By measuring the time‐dependent optical reflectivity, we have investigated the dynamic behavior of annealing with the 30‐psec pulse train of a mode‐locked Nd : YAG laser. It was first observed that at narrow ranges of high laser energy density, the reflectivity of implanted Si and GaAs increases slowly to the level consistent with liquid ones, except GaP, and remains at that level for a period less than 200 nsec. As to Si, the mode‐locked laser is confirmed to have a weak effect on temperature rise and, therefore, to produce a thin molten layer compared toQ‐switched Nd : YAG lasers.