The invention of computer-based writing program has revolutionized the way of teaching second language writing. Embedded with artificial intelligence scoring engine, it can provide students with both immediate score and diagnostic feedback on their essays. In addition, some of such programs offer convenient writing and editing tools to facilitate students' writing process, such as MyAccess. However, the effectiveness of using such programs to improve students' writing skills in the L2 is not conclusive yet. To gain a deeper understanding of how such a computer-based writing program works in EFL writing classes, this study explored factors that may lead to facilitation or frustration when the program MyAccess is in use. The participants were three EFL writing classes consisting of 68 third-year English majors in a Taiwan's university. Data included students' responses to a questionnaire, writing samples, feedback given by MyAccess, and focus-group interviews with the students. The findings show that only 55% of the students felt this program either moderately or slightly helpful to them in improving their writing skills and that the students were much less satisfied with the grading function than the writing/editing functions. Their dissatisfaction was largely because this program failed to give specific feedback in the content and rhetorical aspects of their writings. Moreover, the study also found that the different ways the three writing classes used this program and the instructors' familiarity level with the program coupled with their technological skills greatly affected the students' attitudes toward this program. The study concludes that pedagogical approaches and contextual factors are more crucial than the functions of the program in determining its effectiveness. (Contains 5 tables and 1 figure.)