Abstract: Natural gas jet fire can cause destructive damages by the heat it radiates to the surrounding space. With CFD code, numerical simulations were carried to analyze geometry and heat flux properties of natural gas jet fires. Six cases with mass flow rate up to 163 kg/s, pressure up to 12 MPa, and with outlet diameter up to 100 mm were conducted, and compared with Thornton model results. Flame sizes are obtained to show a linear relationship between outlet diameter and both flame length and width. The calculated results are in a nice fit with the Thornton model results, while heat flux distributions differ when concerning areas are near the outlet. The far field heat flux distributions perform generally the same with engineeringly acceptable tolerance, confirming both methods can be considered in natural gas facility safety designs.