The noncollinear SHG method has been used to measure the pulse width of passively mode-locked Nd: YAG laser. The nonlinear ADP crystal is oriented in such a way that phase matched SHG is produced only when both incident beams are present as ordinary rays. The angle between the two beams is 25°50′. Phase-matched SHG as an extra-ordinary occurs at an angle bisecting the two beams. The correlation width obtained with passively mode-locked Nd: YAG corresponds to a pulse width of 24 ps.