Some features of the closed aperture Z-scan transmittance curves of nonlinear optical materials with Gaussian beam have been studied theoretically when the nonlinear refraction and the nonlinear absorption coexist. It is found that the peak valley or valley peak structure depends on the ratio ρ of the real part to the imaginary part of the third order nonlinear susceptibility. The exact critical value ρc, which determines whether the peak valley or valley peak structure exists or not, is given; the ρ dependences of some parameters of the transmittance curves are obtained numerically, and the scope of applications for the results is also discussed. The Z-scan experiment of CS 2 is consistent with the theoretical results obtained here.