【Objective】 To estimate the heritability of temperament in Chinese school-age twins. 【Method】 Childhood temperament was assessed in a total of 127 pairs of 8 to 12 years old twins with the Middle Childhood Temperament Questionnaire (MCTQ),and structural equation modeling was performed to evaluate the heritability of the category and nine dimensions of temperament. 【Results】 The best-fitting model indicated that 30.03% to 90.40% of the variances of the temperament were attributable to genetic factors,with the remaining variances being due to the effects of common family environmental factors,and the environmental experiences unique to each child were negligible in most temperament traits.The genetic factors (heritability) explained about 50.57% of variance in temperamental category.The heredity of the temperament dimensions in Activity,Rhythm,Withdrawn/approach,Adaptability,Intensity,Emotion,Persistence,Distractibility,and Threshold were 39.30%,46.26%,82.22%,44.31%,41.84%,90.40%,72.46% and 30.03% in the boys,and 88.39%,60.65%,82.23%,44.31%,52.05%,90.40%,72.46% and 44.99% in the girls respectively.The gender effect was found in the heredity of activity.The effects of genetic and environment on the temperament in school-age children was not influenced by the factor of age. 【Conclusions】 The temperament of school-age children is influenced by both the genetic and environmental factors.There are gender effects on the heredity of activity,with more genetic effect on girl's "Activity".