By distinguishing the uncertainty of base earnings ratio and traffic flow, the paper establishes an economic evaluation model for highway projects, which is based on the probability analysis and the Monte Carlo random sampling technology. The application of this model was specified using a preliminary assessment of a highway project case. The results of the case study show that the pre-tax and the after-tax profit objectives of highway projects differ greatly, and it is a great probability event for the after-tax profitability to exceed its target value.