We investigated nonlinear crystals for the second harmonic generation of Nd: YAG laser radiation. KTP and LBO devices were used to generate the secondharmonic light of CW pumped Q-switched Nd: YAG laser radiation. We compared 6kinds of devices. To obtain the second-harmonic-output power as high as 10W, we found that a hydro-thermal-grown KTP device oper-ated at elevated temperature and a flux-grown LBO device operated in noncritical phase match-ing give rise to reliable operation.