Abstract: To set up models for ‘pin-by-pin’ facilities (reactors) and human phantoms with millions of cells by using the MCNP code, the MCNP code was modified and limits on the number of cell, surface, material, etc. were extended. The Snyder head phantom for boron neutron capture therapy was modeled. Neutron and induced gamma-ray absorbed dose in human brain was calculated. Besides, a critical calculation model of repeated structures described by the ‘Like n But’ cards was calculated employing both series and parallel computing. As a conclusion, modifications of the MCNP code were proved to be correct.