Since Wilson (1948) had firstly developed steady-state flow rate and moment loss model for hydraulic piston machines, several authors tried to make precise performance models. However, Huhtala (1997) discussed the strength and weakness of some existing models comparing with measurement data and concluded unfortunately that any model is not accurate enough for wide operating ranges. For a hydraulic axial piston motor of swash plate design with rotating cylinder, new performance formula is derived in this paper through theoretical study on leakages and friction losses of three facing gaps and other non-negligible losses. And efficiency surface of an example motor is estimated and extension of the formula for hydraulic pumps is discussed. A novel feature of the derived model is that all coefficients are given by the physical dimensions of a motor, hence allowing calculation and analysis of the performance of a motor in mind.