This research was an investigation of the application of genre instruction to the unique purpose teaching Thai secondary students to write personal statements for undergraduate university admission. The goals of this small-scale case study were to 1) investigate the effects of genre-based instruction on students' writing abilities and 2) examine students' attitudes toward this instructional method. The participants were eight students in grade 12, four defined as high-ability and four as low-ability. The students completed a didactic sequence developed for this study, beginning with the pre-test and concluding with a comparable and administration of a questionnaire and individual interviews with the participants. The test results revealed that while the overall scores for the entire group showed slight improvement in the students' writing abilities, it was due to the noticeable improvements in the low-ability students' scores, whereas those of the high-ability group decreased. Qualitative analysis of their work revealed interesting but varied improvements in both groups. The attitude results from the questionnaire and interview data were positive for both groups, yet some concerns were also raised. The study demonstrates both the benefits of and concerns about applying the genre-based approach to this specific context while asserting its possibility for further development.