Measurements show the detection efficiency of the NaI(Tl) detector depends on the source-detector configuration. In this work, the geometrical and intrinsic detection efficiencies of the 2in both in height and diameter and 3in both in height and diameter cylindrical NaI(Tl) detectors have been calculated for low-energy gamma ray. In these calculations, the effects of cover layers of NaI(Tl) (aluminum housing and reflector) have been taken into consideration. To evaluate the calculation model, the detection efficiency of the detector is measured and modelled using MCNP simulation tool. Isotropic 241Am was used as a point source. Results show the calculation model is in good agreement with the experiment and Monte Carlo simulation.