Phase conjugation process at intracavity degenerate four wave mixing of a long pulse CO2 laser radiation in its active medium has been studied experimentally. Dependencies of energetic and temporal characteristics of the phase conjugation signal upon specific energy stored into electric discharge, Q-factor of laser resonator, laser mixture composition and pressure, interaction angle, optical delay between probe and co-propagating waves, intensity ratio for these waves J3/J1, a degree of linear polarization, and laser spectrum are analyzed. Reflectivity of the phase conjugation signal is shown to increase with specific input energy rise and a decrease of the ratio J3/J1, being up to 7.5%. An increase of nitrogen content and a decrease of helium content in the laser mixture results in reflectivity rise and peculiar laser pulse profile with two maximums being due to amplitude and phase nonlinearities of the active medium. Specific input energy rise and an increase of Q-factor leads to a decrease of response time for the phase conjugation signal. Nonlinear properties of degenerate four wave mixing on amplitude gain and phase thermal diffraction gratings in the active medium of a long pulse CO2 laser are analyzed theoretically. An interconnection of nonlinear parameters of the amplifying media with the temporal behavior of phase conjugation reflectivity is determined. A comparison of the theoretical calculations and the experimental data has been done.