Prediction of power efficiency of gear boxes has become an increasingly important research topic since fuel economy requirement for passenger vehicles are more stringent, due to not only fuel cost but also environmental regulations. With this circumstance, automotive industry is dedicatedly focusing on developing high efficient transmission. Thus, the analysis of power efficiency of gear box should be performed to have a transmission highly efficient as much as possible at the beginning of design stage. The prediction of the transmission efficiency is a process to understand and calculate the power loss mechanism which can be categorized in two groups such as load-dependent power losses and load-independent power losses. Load-dependent power losses known as mechanical power losses are caused by friction under loads while load-independent power losses known as spin power loss are induced by fluid drag between two bodies with relative velocity. In this study, a program is developed to analyze the efficiency of an entire gearbox, considering all components’ losses such as gear mesh, wet clutches, bearings, oil pump and so on. The analytical models are based on formulations of each component power loss model which has been developed and published in many existing papers. The program includes power flow analysis of both of a parallel gear-train and a planetary gear-train. The program is also implemented with the assumption that the components of power losses are independent of each other. The developed program is employed to estimate the power losses and their efficiencies of several automobile gear boxes. The results are then validated by comparing with the experimental data.