In large-scale monthly unit commitment (MUC), the generation system is committed to a schedule list of available generating units and available fuel provision for each day in a whole month. Despite challenging numerous operational restrictions, fluctuating factors such as load demand, fuel price affect the decision-making on generating electricity in the uncertain market. A Fuzzy Mixed Integer Linear Programming Model (MILP) focusing on the uncertainty fuel price for the MUC in the national level power system is introduced. The pricebased uncertainty is handled by attempting to minimize the fluctuation tolerance of fuel price. Moreover, a solution method for solving Fuzzy MILP is presented. An actual national planning level in Thailand’s power system comprises multiple types of generators, and supplied fuels are tested using the proposed technique. The small test case, including a 24-unit system, is used for sensitivity analysis on fuel price. The national-level planning data are from the Electricity Generating Authority of Thailand (EGAT), with a 204-unit system is conducted to demonstrate the efficiency of the proposed Fuzzy MILP model.