Abstract: The variation regularity of dislocation densites of some deformed steel sheets under three kinds of strain paths as uniaxial tension, plane strain and equal biaxial stretching was investigated by means of positron lifetime spectrum and Doppler-broadening techniques. The results show that the increasing rate of dislocation densites is related to the plastic strain path. A relationship between the increasing rate of micro-defects, macro-instability and forming limits during the process of steel sheet forming was got based on analyzing the mechanism of dislocation density increasing under different stress-strain condihons.