The primary objective of this study was to investigate the role of content knowledge on topic choice in writing. Children's knowledge on topics they wanted to write about (want topics), on topics they did not want to write about (don't-want topics), and on topics the teacher chose (teacher topics) was measured using Langer's topic-specific knowledge measure. Results showed that children had significantly more knowledge, as assessed by the fluency and combined knowledge measures, on the want topics compared to the don't-want topics. In addition, children had significantly more knowledge on the want topics than on the teacher topics. There was no significant difference in knowledge between the children's don't-want topics and the teacher topics. These findings not only demonstrate the significant role of content knowledge on one writing process, topic choice, but also add support to students' self-selection of writing topics.