Although a number of studies have been conducted to explore how various individual difference (ID) factors such as aptitude, motivation, and anxiety contributing to L2 learners' speech performance over a period of time of L2 learning, only a few examined the effect of the three ID factors on learners' L2 speech performances based on classroom language learning setting in a longitudinal study (e.g., Kermad, 2018; Saito et al., 2018; 2019). There are no existent studies that have examined the concurrent influence of aptitude, motivation, and anxiety on L2 speech development with the same group of adult EFL learners as participants in one single longitudinal research project, while all the three ID factors have been demonstrated to be significantly correlated with L2 speaking acquisition (Li, 2016; Matsuda & Gobel, 2004). For filling such research gap, the current study examined a group of Chinese EFL college-level learners' (N = 83) L2 speech development and how different learners' individual difference (ID) factors (i.e., aptitude, motivation, and anxiety) playing a role in determining their L2 speech learning outcomes after they experience one academic semester's L2 speaking classroom-based training. Specifically, the study measured the learners' L2 speech performances at two separate times over one academic semester: before students started to take the first lesson (Time 1); after they finished taking the last lesson of the enrolled English-speaking course (Time 2). The study adopted a sentence reading task and a pair of picture description tasks for examining participants' L2 speech proficiency at a controlled and spontaneous state, respectively. Several types of L2 speech constructs were examined in the study, including speech comprehensibility, accentedness, accuracy, fluency, complexity (CAF), and overall speaking proficiency. The participants' oral performance of accentedness, comprehensibility, and overall speaking proficiency shown in the sentence-reading and picture description tasks were rated by four listeners who were currently-enrolled graduate native English speakers. Additionally, the participants' audio recordings of picture description were transcribed in a verbatim manner, and the indexes of CAF were calculated and coded by the researcher. In terms of the instruments used for examining learners' aptitude, motivation, and anxiety levels, the study adopted L2 Self Guides (Papi et al., 2019), an adapted L2 speech learning anxiety scale (Cheng, 2017; Woodrow, 2003), and LLAMA test (Meara, 2005) to respectively measure learners' motivation, anxiety and different types of aptitude level. After the data were collected, paired-samples t-tests and a set of multiple regression analyses were conducted to respectively examine the learners' L2 speech gains and the predictive power of aptitude, motivation, and anxiety on their longitudinal development of English speech proficiency. The results indicated that after one semester's English-speaking training, the EFL learners in the study achieved significant gains in L2 speech proficiency in terms of accentedness, comprehensibility, accuracy, fluency, and overall L2 speaking proficiency. On the other hand, several ID factors were demonstrated to be associated with different aspects of L2 speech outcome. Mainly, implicit aptitude had a positive effect on speech comprehensibility, accuracy, and overall speaking proficiency. Besides, Ideal L2 Self Own, Ought-to L2 Self Own, and Ideal L2 Self Other appeared to be the most prominent motivation factors that had a positive effect on learners L2 speech gains in different aspects (i.e., comprehensibility, accentedness, accuracy, and overall speaking). On the contrary, anxiety overall exerted a negative effect on L2 speech performances. The study shows its uniqueness, which has separately explored how each of the ID factors (i.e., aptitude, motivation, and anxiety) influences L2 speech development and considered how these cognitive-psychological factors are macro factors associated with and interpreting learners' L2 speech development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]