Background: Monte Carlo (MC) modeling of a Linear accelerator is a prerequisite for Monte Carlo dose calculations in external beam radiotherapy. In this study, a simple and efficient model was developed for Elekta SL-25 Linear accelerator using MCNP4C Monte Carlo code. Materials and Methods: The head of Elekta SL-25 linac was simulated for 6 and 18 MV photon beams using MCNP4C MC code. Energy spectra and fluence distribution of photons crossing the phase space plane were calculated. A simple point source model was developed based on calculated photon spectra and spatial distribution. Using this model, percent depth doses (PDDs), and beam profiles were calculated for different field sizes. The results of MC calculations were compared with measurements. Results: There was a good agreement between MC calculations and measurement for descending part of PDD curves. However, comparing calculated PDDs with measurement showed up to 10% differences for build up region of PDD curves for both energies. For beam profiles, there was 2% difference in flat region and up to 15% difference was seen for out of field region. These results were acceptable according to the recommended criteria. Using this model, the run time was decreased 24 times in comparison to original full Monte Carlo method. Conclusion: Our study showed the presented model to be accurate and effective for MC calculations in radiotherapy treatment planning. Also, it substantially lowers MC runtime for radiotherapy purposes.