Objective  To investigate the associated factors leading to myopia in primary school students in a relatively developed region in Hangzhou. Methods  This was a cross-sectional study. By cluster sampling， primary students from 5 schools （grades 1-6） in Zhaohui District， Hangzhou， were recruited for this investigation. The right eye was examined and data were collected on spherical refraction， cylinder refraction， axial length and corneal curvature. Information was also obtained on family history， time spent on near reading after school and on weekends， and time spent on outdoor activities both after school and on weekends. A one-way ANOVA was used for analysis. Results  There were significant differences in gender， family history， time spent on near reading and outdoor activity between myopic and non-myopic groups. Females were more likely to be myopic than males （63.7% vs. 67.4%）. The students with myopic parents （F=10.559， P＜0.05） or those who spent more time on near reading on weekends （F=7.555， P＜0.05） and after school （F=2.492， P＜0.05） were more likely to be myopic. On the other hand， outdoor activities both after school （F=11.351， P＜0.05） and during weekends （F=6.387， P＜0.05） reduced the prevalence of myopia. Conclusion  Myopia in primary school students is associated with gender， family history， near reading and outdoor activity. So both genetic and environmental factors contribute to the development of myopia. The effect of environmental factors， especially near reading， is time-dependent， and ultraviolet exposure during outdoor activity plays a significant role.