This paper puts forward a valuation model for the flexibility of revamping urea plants based on the substitutability of switching between the intationally accepted fertilizer feedstock fuel oil and natural gas. The paper first builds a stochastic mean-reversion model for fuel oil prices and natural gas prices respectively, then estimates and tests the parameters using the fuel oil and natural gas prices data from the U.S. market. This paper also conducts a sensitivity analysis with important parameters. The results show that the real option method can be applied successfully to evaluate the flexibility in decisionmaking for revamping installations in fertilizer plants or other similar plants.