This thesis discusses the Cerntainty Analysis of Operability Natural Gas Liquefaction in PRICO process using Process Simulator software and Random Simulator Generator software. PRICO process is simulated to be 4 cases, the Base Case, Case 2, Case 3, Case 4 using Process Simulator software. Optimal conditions of operability of the process is done by varying the temperature of refrigerant out and the temperature of Natural Gas out of the Main Heat Exchanger. Optimal conditions of operability of the process in terms of benefit based solely on refrigeration cycle operating costs. Optimal conditions of each case is calculated the level of certainty by using Random Simulator Generator and generate the minimum profit and the level of certainty.