Objective To explore whether hyperbaric oxyGen could improve microenvironment of transplanted stem cell in myocardial infarction rats or not. Methods Sixty-six SD rats were randomly divided into four Groups:sham operated,myocardial infarction( MI),Stem Cell Transplanted( SC),hiGh oxyGen pressure ( HBO)+SC Groups respectively. MI rat model was established,and then HBO treatment was carried out every day from the first day to 30th day. Human umbilical cord Wharton jelly mesenchyme cell was transplanted on the 7th day after MI. The serum level of superoxide dismutase( SOD ) and nitric oxide( NO ) were assayed respectively by xanthine oxidase method and nitric acid reduction colorimetric technique on the lst,7th,l4th and 30th day after MI. Human Whartonˊs jelly stem cells was marked with electronic display unit for tracinG in vivo. Stem cell proliferation and survival rate were identified by immunofluorescence method. Results ( l ) Compared with MI and SC Group respectively,the level of SOD increased siGnificantly in HBO +SC Group( P<0. 05).(2)Compared with MI Group,the level of NO increased siGnificantly in HBO+SC Group all time( P<0. 05),and compared with SC Group,the level of NO also increased siGnificantly at the l4th and 30th day( P<0. 05).(3)Compared with SC Group,the stem cell proliferation and survival rate all increased siGnificantly in HBO+SC Group at the l4th and 30th day(55. 00% vs 80. 58%,34. 28% vs 79. l2%,P<0. 05). Conclusion Stem cell transplanted assisted with hyperbaric oxyGenation could siGnificantly increase the level of NO and SOD,so as to improve the stress state after MI and promot the proliferation and survival rate of stem cell.