Aim Promoting cardiac regeneration is therapeutic strategy to be developed urgently.In present study,we investigated the effects of overexpression of microRNA-99a（miR-99a） on cardiomyocytes proliferation of neonatal mice as well as its potential mechanism.Methods Cardiomyocytes of neonatal mice were infected by lentiviral-miR-99a（LV-miR-99a）.Cardiomyocytes proliferation were identified by thiazolyl blue （MTT） and 5-ethynyl-2’-deoxyuridine （ EDU） assay.Western blot was used to assess the protein expression of Erk1/2 and its phosphorylation levels in cardiomyocytes.Results MTT assay and EDU assay both showed that cardiomyocytes proliferation were significantly higher in miR-99a over-expression group than in the control group （P<0.05）.The phosphorylation levels of erk1/2 was also significantly higher than that in the control group （ P<0.05）.Conclusions miR-99a overexpression can promote cardiomyocytes proliferation.This effect may be partly mediated through activation of erk1/2.