One of the most important requirements imposed on planetary rovers is the ability to minimize slippage while climbing slopes covered with loose soil. Therefore, at the design stage of the rovers, it is necessary to evaluate the traveling performance of their locomotion mechanisms over a slope. However, to conduct traveling tests over slopes, the sandbox in which the tests are conducted must be tilted; it is difficult and even dangerous to make a steep slope because of the heavy weight of the sandbox. Therefore, in this paper, we propose a new method for estimating the traveling performance of a wheeled rover over a slope. In this method, we estimate the slip ratio for different slope angles on the basis of traction tests over a flat terrain. To verify the proposed method, we conducted slope-climbing tests and traction tests in a sandbox using a two-wheeled rover with various types of wheels. Furthermore, we compared the slip ratios estimated from the slope-climbing tests with those estimated from the traction tests. From these comparisons, we concluded that the proposed method can accurately estimate the traveling performance of a planetary rover over a slope.