In this paper, we report out experimental results of optical second harmonic generation from clean Si(100)2×1),Si(100)(2×1)-H and Si(100)(3×1)-H surfaces.For the clean Si(100)-(2×1)surface and the Si( 100)(3×1)-H surface,the second harmonic Intensity decreased monotonlcall with increasing temperature.For the(2 ×1) monohydride Si(100)surface,the second harmonic intensity varied on the temperature not monotonkically,but has the maximum at about 470 K.Structure and sample temperature could be monitored by the second harmonic intensity and the tempeerature dependence of second harmonic intensity.