Optical nonlinearity of laser heated Si with thickness of 160$mu$m is investigated by pump-probe method. Si is heated by pdsed Nd:YAG laser of $TEM_{00}$-mode with pulse duration of 180$mu$sec. Temperature change is monitored by observing transmittance change of probe light whose wavelength lies near band gap of Si. It is found that temperature rise is linearly proportional to incident laser intensity and the maximum temperat~re increment is measured to be $16^{circ}C$ for the maximum incident intensity of 25KW/$ extrm{cm}^2$. From these results, the third order nonlinear susceptibility .d3) at the wavelength of 1.06$mu$mis estimated to be $6.6 imes10^{-5}$esu due to laser heating.