The blended learning model, which combines the traditional face-to-face learning method with an online application such as a learning management system (LMS), became popular and more practical for both teachers and learners in foreign and second language education because of its effective methodology for course delivery and socialization opportunities with technology-enhanced learning activities in both online and offline environments. Although the effectiveness of blended language learning models and benefits of student achievement and autonomous learning with an LMS have been explored, prior research resulted in conflicting data on blended instruction identifying the inconsistent findings in student achievement. Some researchers found that students in blended learning improved their language skills and had higher achievement than participants in exclusively face-to-face or online learning, while some researchers found there to be no statistically important differences in achievement when the blended model was used compared to a face-to-face setting. The specific problem is that the low language achievement of students seems to be related to lack of autonomous language learning skills, but their perceptions of the blended language courses regarding language achievement and autonomous learning skill have not been previously identified and analyzed. The purpose of this qualitative case study is to investigate student perceptions of course effectiveness factors for language proficiency as well as learner autonomy in a blended Korean language course to improve language achievement, especially in listening and reading comprehension skills. American students who attended intermediate and advanced blended Korean language courses applied with the LMS, "SAKAI" in 2014, 2015, and 2016 at the Osan Language Training Detachment (LTD), Defense Language Institute Foreign Language Center (DLIFLC) in Korea, were invited to participate in the anonymous, open-ended online survey. Data from 10 of the participants were analyzed and evaluated. This study found the blended Korean language course was effective for language learning and achievement, but only 50% of participants stated it was effective for the improvement of autonomous learning skills. The other responses said those skills were dependent on various elements of the blended course such as activities, curriculum, teacher, and student's motivation and learning styles. The factors students found to be effective and ineffective as well as suggestions offered to improve the blended language course were discussed. Although the focus was on one Osan LTD teaching Korean language courses at DLIFLC, the findings could be able to improve language achievement and autonomous learning for future learner success as well as curriculum design in other foreign language courses in DLIFLC or other institutions. [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.]