In this study, the response functions of BF3 and 3He neutron proportional counters were estimated using the general purpose Monte Carlo simulation code, MCNPX. Open Pool Australian Light-water (OPAL) research reactor core was simulated and used as case study, and parameters such as the effects of increasing tube pressure and tube diameter on response functions were also investigated. It was observed that the pulse height spectrum for BF3 counter had two sharp peaks at 2.31 MeV and 2.79 MeV, whereas that of 3He counter was seen to have only one sharp peak at 0.76 MeV. The response matrices were observed to increase with increasing tube pressure and tube diameter. Also increasing the tube pressure and tube diameters were seen to increase pulse heights, and reduce the wall-effect phenomenon.