First results and analysis of second harmonic generation with a Nd:YAG laser working in the long pulse, free running mode are presented in this paper. Second harmonic generation conversion efficiencies of up to 17.5% and pulse powers of 162W have been generated with a free running Nd:YAG laser and a KTP non-linear crystal. The conversion efficiency is limited by a saturation effect and optical damage occurring at similar to50 times lower peak intensities than in the Q-switch mode. The saturation and damage mechanism involves creation of temporary 'color centers' induced by the second harmonic radiation and subsequent increased fundamental wave absorption.