A theoretical analysis was used to evaluate the effect of body mass on the mechanical power cost of cross-country skiing and roller skiing on flat terrain. The relationships between body mass and the power cost of overcoming friction were found to be different between cross-country skiing on snow and roller skiing. Nevertheless, it was predicted that the heavier skier should have a lower oxygen cost per unit of body mass for roller skiing, as is the case for snow skiing. To determine whether the theoretical analysis was supported by experimental data, oxygen consumption measurements were performed during roller skiing by six male cross-country ski racers who spanned a 17.3-kg range in body mass. The theoretical analysis was supported by the experimental findings of decreases in oxygen consumption for each kg increase in body mass of approximately 1.0% for the double pole technique, 1.8% for the kick double pole technique, and 0.6% for the VI skate technique.