One of the important factors of a nuclear reactor core is the fast fission factor. This paper calculates this parameter based on space and energy-dependent method using the PTRAC card of MCNPX code. Tehran research reactor (TRR) is taken as a case study, and the parameter analyses are performed on the reactor core. Fast fission factor in TRR is evaluated regarding temperature effect, control rod positions, and fuel assembly positions. Using the PTRAC card, helpful information on fast fission factors is achieved throughout the reactor core. One MCNPX runs to return a data file about neutron interaction that can be analyzed many times in different manners to reveal this useful information. The method is simple and can be applied to any nuclear reactor core. The results obtained by this method can help nuclear reactor designers and nuclear reactor fuel managers to have a precise evaluation of the parameter. The method proposed in this paper for fast fission factor calculation is compared with the results previously published in the literature.