The aim of this doctoral thesis is to apply fundamental data to the developments of the urea-SCR aftertreatment system, and the integrated DOC and urea-SCR aftertreatment system for reducing NOX emissions in a LPG steam boiler. When NOx emissions are reduced by NH3 in an SCR reactor, NOx conversion rate is increased if the fast SCR reaction with 1 : 1 mixture of NO and NO2 is formed. The upstream DOC in an SCR reactor can be used to oxidize NO to NO2 emission. The performed research contents are summarized as follows. In the first study, to investigate the performance characteristics of the urea- SCR reactor, NOX emissions and exhaust gas temperatures in front and at the back of SCR reactor before and after urea injection are experimentally measured by using a flue tube steam boiler with a urea-SCR reactor system operating at four kinds of boiler loads. NOX conversion rates are obtained by 89, 85, 77 and 79% in cases of the boiler loads of 100, 80, 60 and 40%. NOX conversion rate can be raised with increasing the urea injection rate, but NH3 slip is boosted at the same time. Consequently, to determine the appropriate rate of urea injection or the high NOX conversion rate for the respective boiler load, NOX emissions, NH3 slip and reaction temperature should be simultaneously considered. In this study, the appropriate urea injection rates of 230, 235, 233 and 231 mg/min are determined by considering NOx conversion rate and NH3 slip simultaneously at the boiler loads of 100, 80, 60 and 40%. In addition, the final NH3 slips are measured by 8.48, 5.58, 11.97 and 11.34 ppm at the same boiler load conditions. In the second study, to investigate the performance characteristics of the urea- SCR reactor, THC, CO, O2, CO2, soot and SO2 emissions except for NOX in front and at the back of SCR reactor before and after urea injection are experimentally measured by using the same apparatus as the first study at four kinds of boiler loads. As a result, THC and CO emissions are reduced quite a lot as the boiler load is increased, and the effects of catalyst in the SCR reactor on THC and CO emissions are varied depending on the boiler load. In addition, O2, CO2 and SO2 emissions except for soot showed the influence of the boiler load, but the catalyst in the SCR reactor are not affected by all of the O2, CO2, soot and SO2 emissions. In the third study, the characteristics of NOX emissions and exhaust gas temperatures in front and at the back of DOC aftertreatment system before urea injection, and at the back of SCR reactor after urea injection are experimentally investigated by using the same apparatus as the first study at four kinds of boiler loads. It is found that NO emission is much converted into NO2 by the DOC aftertreatment system and it contributes significantly to the rise of NOX conversion rate. NOX conversion rate is greatly increased, particularly at low loads, because the reaction temperature ranges of four boiler loads are raised up to 250 ~ 347℃ by insulating the exhaust pipe connected to the DOC and urea- SCR aftertreatment system, and the raised temperatures correspond to the optimal catalytic reaction temperature ranges of 250 ~ 350℃. NOX conversion rates are significantly obtained by 100, 96, 90 and 89% at the boiler loads of 40, 60, 80 and 100% because the NO emission is much converted into NO2 by the DOC aftertreatment system. NOX conversion rate can be raised with increasing the urea injection rate, but NH3 slip is boosted at the same time. Consequently, to determine the optimal rate of urea injection or the high NOX conversion rate for the respective boiler load, NOX emissions, NH3 slip and reaction temperature should be simultaneously considered. In this study, the urea is injected by 14, 19, 20 and 18 mg/min at the boiler loads of 40, 60, 80 and 100%, and the final NH3 slips are measured by 20.05, 18.35, 9.29 and 13.92 ppm at the same conditions.