The nonlinear optical phase conjugation effect of F_3~+ centers in LiF crystals is observed using degenerate four-wave mixing technique. Using a pulsed dye laser as pump light, a maximum phase conjugate reflectivity of about 0.1% is obtained, The physical origin of the nonlinear saturation absorption and the four-wave mixing in F_3~+ center system is pointed out in theory according to the energy level propertry of the F_3~+ center in LiF crystals.