Precise assay of uranium enrichment without time-consuming analysis is a crucial factor in nuclear safeguards process. This paper aims at a computational development using innovative Monte Carlo (MC) method for 235U mass enrichment (%E) verification for the UF6 cylinder. This new approach focuses mainly on using different UF6 physical properties (effective volume and density) and extra 235U gamma- and X-ray transitions as input parameters for efficient calculations of %E assay. In this work, we used the measured values of the dose rate (μSv/h) due to the emitted gamma- and X-rays of 235U content at the cylinder external surface for enrichment calculations. The attenuation of the main 235U gamma- and X-energies due to the cylinder wall (5B-Type Ni Inconel alloy) was also estimated to wide range of energies using XCOM: Photon Cross Sections Database software. Using this suggested model for 235U enrichment calculation, the calculated value of 19.46 ± 1.28% is within one standard deviation of the certified value of 19.75 ± 0.40%. The mass of 235U was also estimated and found to be 2.6814 kg. Based on this improved approach, the total uranium activity of the investigated UF6 cylinder was calculated and found to be (5.52 GBq), which is 98.6% from the declared activity value (5.6 GBq). These accurate and confident calculated values are direct functions in the improved parameters and the developed MC code.