The well-known MCNP code was used to simulate the NaI(Tl) Detector Response Function (DRF) with 2x2, 3x3 and 5x5 crystal dimensions for gamma rays from 241Am, 137Cs and 60Co sources. Then a code named RNAIGA has been slightly written by us which fitted the Gaussian spectra on the generated Pulse-height spectrums by MCNP code (Monte Carlo N-Particle Transport Code System). The generated spectra by the Monte Carlo were compared with the experimental spectra. In general the agreement between the simulation and the experimental response function was good and at the end the effect of Detector Dimensions (DD) on the NaI(Tl) DRF, the Gaussian Standard Deviation (GSD) and the Energy Resolution (ER) were studied for the full energy peak. It was recognized that with increasing the dimensions of the NaI(Tl) detector crystal, (I.) not only the Compton continuum decreased but also GSD for the full energy peak increased and ER decreased, (II) not only at low energies the variation of the response function of the NaI(Tl) detector was very small but also At high energies variation of response function of the NaI(Tl) detector was very large.

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