Taking the second harmoic generation of Gaussian beam in a KDP crystal of type I matching for example, the effects of diffraction and walkoff to doubling conversion efficiency have been worked out. The result indicates that the effect of diffraction is very small and can even be neglected. When the input Gaussian beam size is very small, the walkoff is very deleterious to doubling conversion. Along with the enlarging of beam size, the effect of walkoff is much smaller. Finally it can even be neglected.