The association between higher levels of physical activity and the number of physical activities and dementia is of major interest. This study was to examine the effect of physical activity on risk of developing dementia. 10 prospective cohort studies that reported relative risks with 95% confidence intervals for the association of physical activity and dementia risk were chosen in this study. A random effects model was used to calculate the summary risk estimates. The summary relative risk of dementia for associated with physical activity was 0.48 (95% confidence interval: 0.33-0.64), with evidence of heterogeneity (P = 0.000, I2 = 83.50%). Findings of the present meta-analysis indicate that physical activity was associated with a reduced risk of dementia and it is important for preventing it.