The sphere method is a newly developed technique for mesurement of nonlinear optical coefficients in crystals. Based on the theory of aperture function of the focused Gaussian beam SHG, the non phase matched (NPM) SHG process in a crystal sphere is analyzed, and the optimum focus condition of type Ⅰ NPM SHG is discussed. The experiment of NPM SHG in a LiNbO 3 crystal sphere pumped with a pico second laser pulse is reported. The experimental results agree well with the theoretical prediction.