A theoretical study and experimental substantiation of the effects of side diffraction on degenerate optical wave mixing mediated phase conjugation are reported. It was found that when the diffracted beams are not impeded by phase mismatch, side diffraction has a profound effect on the amplification of the probe beam, as well as on the phase conjugation reflectivity. A typical plot of the phase conjugation reflectivity as a function of the pump beam intensity parameter shows that the reflectivity can exceed unity. The peak reflectivity occurs at a threshold value much lower than if the diffracted beam were not included. The corresponding probe beam amplification peaks at the same input parameters. Both plots are obtained for the case of negligible loss. When losses and phase modulations are accounted for, the output phase conjugation reflectivity and forward transmitted probe grow unevenly, and their peaks occur at different values of the input intensity parameter.