Abstract: Objective To estimate the heritability of temperament in Chinese preschool twins.Methods The temperament was assessed in a random sample of 131 pairs of 3 to 7 years old twins with the Carey Behavioral Style Questionnaire (BSQ) 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 21.66%-83.96% of the variances of the temperament were attributable to genetic factors,with the remaining variances due to the effects of environmental experiences unique to the child (4%-36%)for each temperament dimension,common family environmental factors (13%-65%)except for the temperament dimensions of withdrawn/approach,persistence and boys' activity,and age factors (0%-8%).The genetic factors (heritability)explained about 45.96% of variance in temperamental category.The heritability of the temperament in dimensions of activity,rhythm,withdrawn/approach,adaptability,intensity,emotion,persistence,distractibility,and threshold was 35.83%,46.55%,82.67%,64.91%,45.09%,44.00%,83.95%,64.93%,and 40.88% in the boys and 81.14%,21.66%,82.67%,80.39%,67.28%,44.00%,83.96%,64.93%,and 40.88% in the girls,respectively.The gender effect was found in the heritability of activity.The effects of genetic and environment on the temperament in 3 to 7 years old children was not influenced by age.Conclusion The temperament of preschool children is influenced by both genetic and environmental factors.There are more environment effects on rhythm,emotion,and threshold in both boys and girls,and activity and intensity in girls.