Objective：To analyze the expression of costimulatory molecule B7-H3 in patients with acute myeloid leukemia（AML），and explore the clinical significance of the expression of B7-H3. Methods：The expression of B7-H3 protein was detected by flow cytometry（FCM）and the expression of B7-H3 mRNA was detected by RT-PCR. We analyzed the relationship between the expression of B7-H3 and some clinical factors. We evaluated the progression-free survival（PFS）and overall survival（OS）between the B7-H3 positive group and the B7-H3 negative group. Results：The results demonstrated that B7-H3 was weakly expressed on bone marrow of AML patients，and RT-PCR also detected mRNA of B7-H3. No B7-H3 positive cells were detected in the normal human bone marrow. B7-H3 was detected in 28 of the 66 cases（42％）. However，among 66 AML cases，no significantly higher frequency of B7-H3 positive cases was found in the male or female group（P=0.924）. Neither was observed between patients who were older than 60 years and those less than 60 years（P=0.541）. The B7-H3 positive expression was of no significant within the higher WBC subset（≥ 30×109/L）（P=0.310）. There was significant differences between favorable karyotype subset and unfavorable ones（P＜0.001）. There was also statistical difference in PFS（7.4 mon vs. 15.2 mon）and OS（9.8 mon vs. 17.8 mon）between the B7-H3 positive group and B7-H3 negative group，respectively（P＜0.05）. Furthermore，in the AML-M2 subtype，the expression of B7-H3 on the unfavorable karyotype group（8 cases）was higher than the favorable group（29 cases，30.05% ± 23.10% vs. 10.08% ± 7.07%，P＜0.001）. There was coexpression between B7-H3 and CD34 of blast cells. The expression of B7-H3 in the CD34 positive group was higher than that in the negative group（20.90% ± 19.70% vs. 9.63% ± 7.21%，P=0.034）. Conclusion：The expression of B7-H3 in AML has relationship with the expression of the karyotype. The B7-H3 positive patients have the worse prognostic. The research of B7-H3 may be a new prognostic marker and provide insights into the leukemogenesis of acute myeloid leukemia.