Increasingly demanding performance standards and operating requirements are driving the growing interest on the influence of gear oil film on reliability under the conditions of high speed and heavy load. The fatigue life of gears can be increased by optimizing the oil film thickness on the gear tooth surface. The oil film thickness can reflect the ability of the gear to resist pitting and gluing failure. Many theories and experiments are devoted to the study of the oil film thickness. The improvement of gear manufacturing accuracy also makes the influence of tooth surface roughness on oil film thickness not negligible. In order to compile and categorize key investigations in an expansive field with substantial recent research, this work reviews oil film thickness with focus on numerical calculation methods, influence of gear surface roughness on oil film thickness and fatigue life analysis based on oil film thickness.