Abstract: Compostie effects between zinc dialkyldithiophosphate(ZDDP) and succinimide in liquid paraffin were investigated, and the protecting films formed on iron surface by either of the two additives alone or their combination were studied with the aid of in situ dynamic temperature variation infrared spectroscopic technique. By comparison with other additive combinations, mechanism of the composite effects produced between the two additives were explored. The experimental results indicated that a complex is formed as a result of interaction of ZDDP and succinimide, and the complex may take part in the surface film formation process, altering the structure and composition of the film. It causes the film more easier to form than that formed by ZDDP alone. It also elevate the decomposition temperature of the film significantly. Thus, combination of the two additives presents effective antiwear and friction reduction performances, and appears good synergistic behavior. In the presence of ZDDP, the dispersion ability of succinimide deteriorated, due to the competitive adsorption of the complex formed in the friction process, which reduces the adsorption of succinimide on the dispersed particles, leading to the aggregation of particles. Results is this work sugyest adverse effect on dispersive performance between the two additives.