The purpose of this study was to investigate the expression of inducible nitric oxide synthase in trophoblasts and deciduas in early medical abortion, and study the relationship of medical abortion through mifepristone and inducible nitric oxide synthase (iNOS) in early pregnancy. Expression of iNOS in trophoblasts and deciduas was detected by both hybridization and immunohistochemical assay in 40 patients (experimental group); the positive expression of iNOS was represented by number density (N/S) and positive unit (Pu) using computer color magic image analysis system (CMIAS). All results were compared with that obtained from vacuum aspiration. In the experimental group, N/S and Pu in trophoblasts were 0.120 ± 0.010 and 15.3 ± 2.6, respectively, while in the control group, they were 0.021 ± 0.003 and 3.1 ± 0.5, respectively, and there were significant differences between the two groups. By immunohistochemical assay, N/S and Pu were 0.090 ± 0.010, 10.24 ± 1.55 0.016 ± 0.002, 1.26 ± 0.33 in the trophoblasts of the two groups; there were also significant differences between the two groups. There were lower iNOS expressions in deciduas by in situ hybridization and immunohistochemical assay, and the difference between the two groups was not significant.It was concluded that mifepristone induced medical abortion through the expression of iNOS in trophoblasts but not in deciduas.