The premixed combustion system applying exhaust gas recirculation was investigated to achieve the low pollutant emission and the high thermal efficiency. In this study, it was studied the effects of EGR on the thermal efficiency, NOx and CO emissions with various EGR ratios and equivalence ratios. As results, when equivalence ratio was increased, thermal efficiency increased and NOx and CO concentration increased. When EGR was applied, NOx and CO concentration decreased and thermal efficiency increased. Especially, in the case of 15% of EGR ratio at 0.85 of equivalence ratio, NOx and CO concentration will be a smaller than these of a current operating condition of the boiler and thermal efficiency was about 1.7% higher.