This paper provides a methodology to determine the upper and lower limits of concession period of public private partnerships that would be useful both to the public and the private sector with the impact of risks and uncertainties taken into consideration. The model uses Monte Carlo simulation to evaluate the effect of risks and uncertainties on the concession period. Net present value (NPV) analysis is used for financial evaluation. To demonstrate the applicability of the proposed model, hypothetical public private partnership (PPP) rail way project in Egypt was used a case study. After applying the model accurate results are obtained. However, using this model gives the practitioners a clearly vision about the value of concession period insuring that the private sector doesn't lose his investment and the government can benefit from the project before end of its service life.