Two algorithms are proposed for dynamic load balancing which reduce the number of rollbacks in an optimistic parallel discrete event simulation (PDES) system. The first algorithm is based on the load transfer mechanism between lps while the other is based on the principle of evolutionary strategy. Both algorithms are implemented on a cluster of heterogeneous workstations to determine their performance. The experimental results show that the algorithm based on the load transfer is effective when the grain size is greater than ten milliseconds, while the one based on process migration yields good performance only for grain sizes of 20 milliseconds or larger. In both cases, the speed up ranges mostly between 1 and 2 using four processors.