Abstract <p>The ability to precisely determine the level and height of liquids in industrial reactors and vessels that operate at high pressure and temperature plays a crucial role in the petrochemical, oil, and steel industries. Since the exact measurement of fluid or liquid levels is impossible due to high pressures and temperatures in vessels, a technique has been presented to calibrate gamma level gauges. To achieve this aim, the nuclear level gauge of a petrochemical stripper was simulated using Monte Carlo N-Particle eXtended (MCNPX) in real and operational conditions in the oil district in two stages. First, the nuclear level gauge consisting of a source, detector, and vessel (stripper), including water and air for calibration, was simulated with different height percentages. The results were compared, analyzed, and validated with experimental data in operational conditions. According to the results, the mean relative error (MRE%) was less than 6.71% and the root mean square error (RMSE) was predicted to be 0.01. The results showed that the acquired data from the simulation are in good agreement with real data (experimental). Then, the level gauge and stripper containing urea and gases at high temperature and pressure, and with similar height percentages in the first stage, were simulated. The results, which are completely consistent with the experimental findings, were converted into the required format and input into the nuclear electronic system for final calibration.</p>