Based on the assumption of culture determinant of sex differences in children's achievement in elementary school, the following hypotheses of this study were tested: (1) Girls do better on the average than boys in school achievement in female-teacher classes; whereas in male-teacher classes boys do better than girls. (2) Boys and girls are treated differently in school and these differences are related to sex differences of teachers; female teachers show more positive attitude toward girls, whereas male teachers show more positive attitude toward boys. Thirty classes, from first through sixth grades, were chosen from five areas with different ,sex ratio of teachers. These classes were taught by 32 female and 28 male teachers respectively. Students' achievements in Chinese and mathematics were collected and the comparisons were made between boys and girls in each class. A attitude scale with 27 questions developed by the author were filled up by 256 teachers (M, 141; F, 124). The main findings of the study were: (1) Sex differences in the two subjects were related to sex differences of teachers. This result indicated to support the first hypothesis. (2) Boys and girls were evaluated differently by their teachers, but these differences were not found to be related to sex differences of teachers. Teachers on the whole showed more favorable attitude toward girls. The results were dicussed in terms of social pressure, role identification, and social reinforcement.