The results of pulsed laser radiation action (UV region) on the mechanical properties (microhardness, fragility, density, and behavior of dislocations) of GaP crystals are presented. It is established that the action of laser radiation in situ generally results in the hardening of the surface layer of irradiated samples: their microhardness increases. It is also found that the effect of laser irradiation is of long duration (reaching sometimes several days) and causes an aftereffect. During this long process, alternating modifications of microhardness are observed: a decrease and increase in microhardness take place. In this case, the effect of sharp increase in microhardness is revealed: its value exceeds the reference quantity (initial one). The possible mechanisms of these phenomena are discussed.