In the present study, a 3-D Parallel DSMC method in developed on unstructured meshes for the efficient simulation of rarefied gas flows. Particle tracing between cells in achieved based on a linear shape function extended to three dimensions. For high parallel efficiency, successive domain decomposition is applied to achieve load balancing between processors by accounting for the number of particles. A particle weighting technique is also adopted to handle flows containing gases of significantly dirrerent number densities in the same flow domain. Application is made for flow past a 3-D delta wing and the result is compared with that from experiment and other calculation. Flow around a rocket payload at 100km altitude is also solved and the effect of plume back flow from the nozzle in studied.