The piezoresisitive effect of p type diamond films is analyzed by deformation potential theory and valence bands split off model.It is found that the great difference of the efficient mass between the heavy and light hole of diamond is one of the main factors which are responsible for the piezoresistive effect in the p type diamond films.The gauge factor calculation formula is obtained according to the strain induced heavy and light hole band splitting off model,and the calculation results are in accordance with the experimental results.