This paper presents the effect of swirl number on combustion characteristics such as temperature, velocity, gas concentrations in a natural gas diffusion flame. Numerial simulations carried out using the commercial computational fluid dynamics (CFD) code, Fluent by choosing appropriate model parameters. The combustion reaction scheme in the flame region was modeled using eddy dissipation model with one step global reaction scheme. A standard k-epsilon turbulence model for turbulent closure and P-I radiation model for flame radiation inside the combustor is used in the numerical simulations. In order to investigate the swirling effect on the combustion characteristics, seven different swirl numbers including 0; 0.1; 0.2; 0.3; 0.4; 0.5; and 0.6 are used in the study. Numerical results are validated and compared with the published experimental and simulation results. A good consistency is found between the present results and those published measurement and simulation results in the available literature. The results shown that the combustion characteristics such as the flame temperature, the gas concentrations including CO2, H2O, O-2, and CH4 are strongly affected by the swirl number. Depending on the degree of swirl, the fluid dynamics behavior of natural gas diffusion flame including axial velocity distribution, central recirculation zone (CTRZ) and external recirculation zone (ETRZ) were also strongly affected.