The purpose of this project is to design and build an air/fuel premixer for a 5kW natural gas fueled porous media burner. The current burner is equipped with a premixer with poor performance, such that the non-uniformity of its output, results in high values for NOx and CO emission. The design of new premixer is aimed at decreasing the emission of these pollutants by generating a uniform mixture of air and fuel. The task has been achieved in three steps: Design, Manufacturing and Testing. In the design section, firstly, three conceptual designs were introduced and compared by performing CFD simulations and analyzing their results. Considering mixing quality, the amount of pressure loss and the simplicity of manufacturing process, a suitable design was chosen from these three concepts. Then, the chosen design was optimized with respect to the injection angle and geometrical parameters, using CFD and Taguchi method of analysis and design of experiments. Because of its importance and lack of a comprehensive study, the injection angle effect on mixture uniformity was investigated separately. It was shown that the best mixture uniformity is achieved at the injection angle of 165°. In next step, the premixer was manufactured on the basis of the optimized design. Then it was tested for its performance, regarding CO and NOx emission and stability of the burner. For the test runs, the concentration of CO and NOx in the flue gas was measured for burner powers from 2 to 5 kW, and equivalence ratios from 0.5 to 0.7. The measured values for the new premixer was then compared with the old ones. Results show that the new premixer has improved burner stability and reduced NOx concentration in the flue gas which confirms that it has improved mixture uniformity. Effect of new premixer on CO emission depends on the equivalence ratio and the burner power. For lean mixtures and low burner powers, uniformity of the mixture has caused an increase in CO emission due to global quenching, but for higher equivalence ratios and powers it has successfully decreased CO concentration in the flue gas. Keywords: Air/fuel Premixer, Mixture Uniformity, Injection Angle, Taguchi Method, Porous Media Burner, CO, NOx