Stepped planing hulls have the potential to reach high-speeds in the sea. The step on the bottom of these vessels influence the pressure distribution and thus stability of the vessel, especially in transverse plane. Understanding the behavior of these vessels in a non-zero heel condition is fundamental in the early stage design. In the current paper, numerical simulation of the viscous flow field around a heeled one-stepped planing hull is performed to evaluate influences of the asymmetric planing on the performance of the vessel. The numerical model is validated in two steps. At the first step, performance of a heeled stepless planing hull operating in calm water is simulated using the numerical model, and the computed data are compared against experimental data. At the second step, the numerical model is used to compute resisatnce anad running attitudes of a one-stepped planing hull in symmetric condition, and the obtained results are compared against experimental measurements. Numerically computed results are in good quantitative agreement with laboratory measurements, showing that the numerical model has reasonable accuracy. On the whole, it has been found that a heeled condition manages a one-stepped vessel to settle at a smaller trim angle. But, unlike stepless boats, large heel angles have less effects on the trim angle at high-speeds since significant negative pressure is caused behind the step by the air, triggering negative pitching moment. It is shown that the resistance of a one-stepped planing hull is increased up to 25% at most of speeds and heel angles when the vessel advances in asymmetric condition. Moreover, this study shows that, the heeling moment of a one-stepped vessel decreases as the speed increases. The extent of reduction in the heeling moment gets more significant by the increase in heel angle at high-speeds, and, as a result, the heeling moment required to keep the vessel at large heel angle is observed to converge to the heeling moment correspanding to small heel angles.