Objective：To assess the prevalence of thyroid peroxidase autoantibody and relationships between thyroid function and TPOAb in the first trimester of pregnancy. Methods：A total of 611 women during the first trimester from Beijing Friendship Hospital are tested for serum TSH，FT4 and anti-TPO by chemilumino metric immunoanalysis . Women with previous thyroid diseases are excluded. We determine the reference intervals for TSH and FT4 and establish the prevalence of TPOAb by using gestational age-specific interval. The relationships between TSH，FT4 and TPOAb are analyzed. Results：(1)The median value of TPOAb is 38.9 mU/L，ranging from 6.4 to more than1 300 mU/L at 8-12 gestational week .(2)The cutoff value of TPOAb is 206.77 mU/L at the 90th percentile by using gestational age-specific interval. The positive rate of TPOAb is 10.8% (66/611).(3)A simple regression analysis shows that serum titer of TPOAb correlates with the levels of TSH and FT4 (P= 0.000 and 0.000). The median concentration of TSH is significantly greater in TPOAbpositive women than in TPOAbnegative women with increased value of 0.40 mU/L. The risk of abnormally high TSH in former is more than three times than in latter. Conclusion：TPOAb positive rate is 10.8% in first trimester. Determination of the gestational agespecific intervals for TSH，FT4 and TPOAb is of very importance to avoid overestimation of maternal thyroid dysfunction. Pregnant women with TPOAbpositive have a higher level of TSH and should be monitored closely for thyroid function. 