Irradiation of metallic targets by a high-energy pulsed laser can generate in materials shock waves with pressure amplitudes of the same order as with conventional shocks from explosives, flyer plate impact etc., but with much shorter pulse durations. Experiments were performed with a 0.6 ns pulsed laser on 304 austenitic stainless steel samples. The effects of induced pressure on the microstructure were investigated by transmission electron microscopy in addition to microhardness measurements and are compared with the conventional results. The twin density and the presence of alpha-phase are particularly studied. In spite of the very short pulses, twins were present in the observed areas whatever the pressure, while alpha-phase embryos were only present in the pressure range 15-25 GPa.