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Micronuclei tests (MNT) using the Chinese Hamster Ovary (CHO) cells and Chinese Hamster Ovary Mutant cells (XRS5) have been conducted to evaluate the biocompatibility of stainless steel 304 (SS304) and nitrogen plasma-implanted SS 304. SS304 induces high MN ratios of the two cell lines and thus has poor biocompatibility. The MN ratio of CHO cells is higher than background by about 63.9% and the MN ratio of XRS5 cells is even higher by about 100%, suggesting that most of the cellular DNA damages on the SS304 are DNA double-strand breaks (DSBs) and are efficiently repaired by the nonhomologous end-joining (NHEJ) pathway. The surface biocompatibility of SS 304 may be enhanced by nitrogen plasma immersion ion implantation (PIII). The altered oxidized species on the nitrogen plasma-implanted surface block cellular DSB repaired by the NHEJ pathway and decrease the MN ratio of XRS5 cells. Our results suggest that cellular DNA damages on the SS304, Cr and N implantation SS304 are mainly repaired by NHEJ pathway way. While cellular DNA damages can be blocked by SS304 implanted NH3 and N. Our results also suggest that the genotoxicity of cell assay such as MNT and DSB is a valid method to investigate biocompatibility.