Third-order nonlinear susceptibility x(3)(-ω;ω,ω,-ω) is calculated in nearly-degenerate quasi-two-level system by perturbation method. We have studied the level crossing effect in degenerate four-wave mixing and compared it with the level crossing effect in resonance fluorescence. The possibility of using level crossing effect in degenerate four-wave mixing to measure the relaxation rate of upper and lower state is pointed out.