In this study, we present the effects of design parameters on the performance of a hopping robot. In particular, we focus on the effects of torsional stiffness and preload on a hopping model with two-segment legs having passive knee joints with torsional stiffness. One of the objectives for a field robot is the ability to travel quickly with energy efficiency . Therefore, it is important to determine what range of each design variable is proper to realize not only the high forward speed for great mobility, but also the low torque consumption for energy efficiency. Accordingly, we describe both the analogy simulation of a hopping gait with two-segment legs and the results of this simulation.