Although many athletic activities and plyometric training methods involve both unilateral and bilateral movement, little is known about differences in the leg stiffness (K leg) experienced during one-legged hopping (OLH) and two-legged hopping (TLH) in place. The purpose of this study was to investigate the effect of hopping frequencies on differences in K leg during OLH and TLH. Using a spring-mass model and data collected from 17 participants during OLH and TLH at frequencies of 2.0, 2.5, and 3.0 Hz, K leg was calculated as the ratio of maximal ground reaction force (F peak) to the maximum center of mass displacement (ΔCOM) at the middle of the stance phase measured from vertical ground reaction force. Both K leg and F peak were found to be significantly greater during TLH than OLH at all frequencies, but type of hopping was not found to have a significant effect on ΔCOM. These results suggest that K leg is different between OLH and TLH at a given hopping frequency and differences in K leg during OLH and TLH are mainly associated with differences in F peak but not ΔCOM.