It is generally agreed that demotivation is any forces that reduce a student's passion or enthusiasm to learn. Despite some studies on demotivation across language levels, culture, and languages; research has not been conducted on university students from different language levels in Thailand. The aim of this mixed-method study is three-fold. Firstly, it attempts to factorize types of demotivation among English major students. Secondly, it examines the effect of demotivation on EFL achievement. Thirdly, it investigates the differences of demotivation in different levels of EFL achievement. The study involved undergraduate students majoring in English completing a questionnaire and being interviewed. The results revealed that there were four potential types of demotivation among English major students: 1) Media, teaching styles and teacher competence; 2) Attitudes towards teachers and classmates; 3) Experiences of failure and attitudes towards English learning, and 4) Characteristics of lessons and class materials. Findings also showed that Demotivation Type 3 (Experiences of failure and attitudes towards English learning) influenced EFL achievement (p <0.001). Interestingly, there was a significant difference in the degree of influence of Type 3 among low, moderate, and high levels of EFL achievement. In the conclusion, pedagogical implications of these findings are discussed in order to help teachers understand important factors that demotivate students to achieve in English language learning so that the occurrence of those factors may be avoided.