The purpose of this study was to investigate self-determined motivation for extracurricular reading among fifth and sixth graders as well as to explore its relationships with background variables (gender, grade), reading behavior, and reading comprehension. The participants were 925 fifth and sixth graders in Kaohsiung City and Pingtung County. They took the Self-Determined Reading Motivation Scale, the Reading Activity Inventory, and the Reading Comprehension Test. Statistical methods included cluster analysis, test of homogeneity of proportions, and one-way multivariate analysis of variance. We found that: (a) in estimating the fifth and sixth grade students´ self-determined motivation profiles in extracurricular reading, a six-cluster solution emerged; (b) gender differences were found among the six clusters of self-determined motivation; (c) grade differences were found among the six clusters of self-determined motivation; (d) differences on reading behaviors were found among the six clusters of self-determined motivation; and (e) differences on reading comprehension were found among the six clusters of self-determined motivation. The results supported the crucial role of self-determination in students’ motivation for extracurricular reading. Finally, based upon the results, we provided suggestions for reading practices and future studies.