Abstract: Oil soluble stannous naphthenate was synthesized. The chemical structure of the target product was characterized by means of infrared spectrometry, while the Sn content was determined by elemental analysis. The tribological behavior of the resulting stannous naphthenate as an additive in VG26 white oil was examined on a four-ball machine, while its synergistic effect with S- or P-containing commercial additives were also explored on the same four-ball machine. The elemental composition of the boundary lubricating film was examined by means of Auger electron spectroscopy (AES). It was found that the stannous naphthenate as an additive contributed to significantly increasing the anti-wear ability and load-carrying capacity of the base stock but it had little effect on the friction-reducing ability and only had a marginal synergistic extreme-pressure action with the S-containing commercial additive. The improvement of the antiwear and extreme-pressure properties of the base stock by the synthetic additive was attributed to the formation of a boundary lubricating film composed of tin compounds on the rubbed surface.