A new technique for measurement of femtosecond pulsewidth is proposed with equality-width in this paper. The positions on the up-envelope of interference correlation curve are given at which the envelops width equals FWHM of the second order intensity correlation curve. They are 3.984, 4.875 and 4.928 corresponding to pulse shape of sech2t/T, exp(-2t2/T2) and exp (-2t/T), respectively. The critical delay velocities for two kinds of correlation measurement are defined. Theoretical analysis is in good agreement with experiment. The results show that the measurement method oan enhance the resolution of correlation measurement of ultrashort pulse to a few or eyen tens times.