Objective To investigate the development of rat heart and the expressions of TGF-β1,SMAD4 and Bax protein to detect the location and mechanism of action in different developmental periods of rat heart. Methods Histology and immunohistochemistry of rat embryonic hearts from day11 to day19(E11～E19) in paraffinembedded were used to analyze the heart development and TGF-β1,SMAD4 and Bax protein expressions. Results The muscular part of interventricular septum appeared on E12.5,and the partition of the ventricle finished on E16.The positive expression of TGF-β1 can be seen in the rat embryonic heart during E11～E19. The positive staining was increased to E15 and then declined significantly .The expression of SMAD4 was enhanced gradually and the positive signals were strong on E17,and a spatial difference was found in the expression on E13. The expression peaks of the Bax protein appeared on E15 then subsided to a stable.Conclusion The critical period of cardiac muscle cell differentiation and heart moulding was E15. TGF-β1,Smad4