In this study An experimental study of the effect of FGR(flue gas recirculation) on the NOx and CO emission was mainly reported in the various load conditions. Other factors access air ratio and burner configuration were simultaneously considered. The experimental study was performed in a full-scale(maximum $80^5 kcal/h$) boiler fired by a gas burner of industrial type. The data reported include exit gas concentration of $O_2$, CO, and NOx. The LNG fired furnace adapted for FGR shows great performance in the NOx reduction. At least 50% NOx reduction in was measured at 20% recirculation rate. An aim was to find operating conditions characterized by stable combustion with a reasonable level of CO emissions and a significant NOx reduction. This was achieved in a range of parameters 1.05-1.10 access air ratio. The flame instability was known as a shortcoming of FGR but flame instability like flame-off was not detected in all cases including over 30% recirculation rate. The flue gas data show a marked decrease of NOx emission with FGR without significant effects on flame stability, overall combustion efficiency and CO emission.