A study was conducted to investigate the effect of gastrocnemius muscle stiffness on achilles tendon force in rebound jumps on slanted contact surfaces. Five trained college jumpers and throwers, and six active males executed five continuous repetition rebound jumps under three surface gradient conditions. The surface gradients were 8 degrees uphill type (U8), 8 degrees downhill type (D8), and level type (L) conditions. Force plate and limb position data were recorded simultaneously during all jumps. The changes in length of the achilles tendon (L_), m. gastrocnemius (L_) and gastrocnemius muscle-achilles tendon complex (L_) during the eccentric phase were calculated according to the mode1 of Voigt et al. (1995) and Grieve et al. (1978). The main results were as follows; 1. Jumping height in U8 and L was higher than in D8. There was also a tendency for a higher achilles tendon force at the midpoint (ATF_) in U8. On the other hand, in U8 and D8, L_ was lower, and L_/L_ was higher than in L. 2. There was significant positive correlations between the ground reaction force at the midpoint (GRF_) and jumping height, and ATF_ and jumping height in U8, D8 and L. 3. There were significant positive correlations between GRF_ and ATF_, and L_/L_ and ATF_ in U8 and L. However, L_lt:MTC> and ATF_ showed a significant negative correlation. These results indicate that the stretch length of the gastrocnemius muscle-achilles tendon complex is lower, and ATF_ higher under uphill-type conditions than under level conditions because L_ becomes higher as gastrocnemius muscle stiffness increases.