Automatic transmissions, which have the advantages of compact structure and smooth shifting, are installed in various vehicles with engines and hybrid power sources. Research and development are continuously being conducted to improve power and fuel efficiency. In this study, the influence of helix direction and helix angle of the planetary gear set on thrust-bearing power loss in an automatic transmission was analyzed. A sample automatic transmission model was constructed to analyze the axial load and bearing relative rotation speed, which are the main factors in thrust-bearing power loss. The relative rotation speed of the bearing was analyzed using the sample model, and the thrust-bearing load in the axial direction was analyzed according to the helix direction of the planetary gear set constituting the model and the helix angle of the planetary gear set. The power loss occurring in thrust-bearing was derived using the analysis results of relative rotational speed and load, and the influence of the helix direction and helix angle of the planetary gear set was analyzed.