Objective To identify the clinical phenotype and gene mutation in two kindreds with type I inherited antithrombin(AT)deficiency.Methods The coagulation and anticoagulation testing and thrombophilia screening were used for phenotypic diagnosis and irmnunonephelometry and chromogenic assay for plasma level of AT antigen(AT:Ag)and AT activity(AT:A),respectively.All of the sevefl exons and intron-exon boundaries and untranslation regions of AT gene were amplified by PCR.and the PCR products analysis Was by direct sequencing.The corresponding gene sites of the two family members and healthy individuals were detected according to the gene mutation sites.Results The plasma levels of AT:Ag of proband 1 and proband 2 were 126 mS/L and 117 mg/L,and AT:A Was 49%and 48%.respectively.Heterozygotic deletion of 3239-3240delCT in proband 1 and uonsense mutation 3206A→T(K70Stop)in proband 2 were rchaaeterized in exon 2 of AT gene.And some of their fmnily members were also detected with the heterozygotic gene mutation.Conclusion Type I inherited antithrombin deficiency of the two probands were caused by AT gene mutation 3239-3240delCT and 3206A→T(K70Stop).