Loss factors of automatic transmission include gear efficiency of oil pump and drag loss of friction factors (Wet type Multidisc Clutch, Brake). Among these factors, this study will observe the potential causes of drag loss, the most influential factor. A simple method is suggested to calculate the drage loss of friction factors in order to predict the power transfer efficiency for a newly designed automatic transmission. The result of a computational fluid analysis program and a free load test are compared in order to verify the suggested method. A calculating of the friction’s drag loss enhances the accuracy by reflecting the inflow air content which is produced due to the relative rotation of friction factors with the existing calculation. The amount of the output of air after applying various grooves to the computational fluid program is then apparent in the calculation. This study aims to improve the prediction of mileage accuracy in order to advance the initial design of an automatic transmission.