Based on elastohydrodynamic Lubrication theory (EHL), the EHL oil film thicknesses and those distributions for two operation cases on meshing line were calculated: one case is the sun gear as the driving gear and another case is the ring gear as the driving gear in the Planetary Gear Transmission. From the graph, gear lubrication can be reflected by the gear mesh lubrication at the node. The worse Lubrication occurred when the planetary gears mesh with the ring gear for the sun gear as a driving gear. By comparing many calculations the lubrication oil film thickness can be significantly increased by the raise of oil viscosity and gear pressure angle. Design on lubrication of the gear to reduce friction and wear is necessary. The increase of oil film thickness can improve the lubrication of gears and prevent wearing and reduce the production cost of gears, which have great practical value.