The research purposes were to: 1) develop lesson plans for content-based instruction; 2) evaluate the students' perceptions of the effectiveness of content-based instruction; and 3) investigate the effects of content-based instruction on English language performance of Thai undergraduate students in a non-English program. The sample group was 19 Thai undergraduate students. The research instruments were: 1) lesson plans for content-based instruction; 2) an evaluation form of lesson plans; 3) an effectiveness questionnaire on content-based instruction; and 4) English language performance tests. Data were analysed the mean, standard deviation, content analysis and a t-test. The research results were: 1) the lesson plans were developed and evaluated by experts as applicable for use at a high level and pilot-tested with 14 non-targeted Thai undergraduate students with the perceived effectiveness at a high level; 2) the 19 targeted Thai undergraduate students perceived the content-based instruction as an effective methodology and essential aid in generating opportunities to use English at a high level. They thought that it was fun and helped them practice, have a better attitude and gain more courage to express themselves in English; and 3) the post-course English language performance were significantly (P < 0.05) higher than the pre-course English language performance. In conclusion, content-based instruction produced positive results and could be used as an effective methodology and essential aid in generating opportunities to use English, which resulted in increased English language performance.