This paper's aim is analysis the factors affecting writing motivation of secondary student writers. I selected 1,192 students out of seven secondary schools and analyzed the data of students' responses on questionnaire for it. The factors affecting writing motivation consists of following four : cooperative interaction, writing self-efficacy, competitive efforts, challenge. They have explanation capability of 57.857% to total variance of the motivation. Cooperative interaction is first factor affecting writing motivation, because of its the highest explanation capability of 38.040%. But differently with preceding studies, writing self-efficacy has low factor loading in writing motivation. The factors affecting writing motivation has differences of gender and grade. The female students gain more scores than the male students. It means that the female students have positive writing motivation. Then 1st grade students gain more scores than others grades 2nd and 3rd. It means that the 1st grade students have more positive writing motivation than others grades. Therefore, as secondary student writers move up, writing motivation goes down.