This paper discusses the algorithm and performance of a 3-D parallel particle transport code system, PENTRAN™ (Parallel Environment Neutralparticle TRANsport). This code has been developed in F90 using the MPI library. Performance of the code is measured on an IBM SP2 and a PC cluster. Detailed analysis is performed for a sample problem, and the code is used for determination of radiation field in a real-life BWR (Boiling Water Reactor). Using 48 IBM-SP2 processors with 256 Mbytes memory each, we have solved this large problem in ~12 hours, obtaining a detailed energy-dependent flux distribution.