The response functions of a 7.62 x 7.62 cm NaI(Tl) scintillation detector to photons from point gamma-ray sources, 10 cm from the scintillator surface, in the energy up to 1.5 MeV, were calculated using the Monte Carlo method, applying simple approximations based on the peak to total ratio and the detector resolution. The Compton continuum of the detector response function was assumed as an isotropic (rectangular) region for the photon energies up to 1 MeV. In the energies between 1 and 1.5 MeV, the Compton continuum was obtained assuming a single Compton scattering with free electrons. The photopeak of the detector response function was assumed as a line. Each determined channel of the response function was distributed to Gaussian functions. The obtained response functions were compared with the experimental values and a good agreement was found.