A novel, simple and feasible method was proposed for increasing intermediate soot concentration and enhancing thermal and radiative characteristics of natural gas flame. In the method, based on pyrolysis technique, a portion of natural gas, as the secondary gas injection rate, was directly injected into the flame to create high temperature and emissive intermediate soot particles. The portion of the fuel dedicated to secondary injection rate varied in the range of 20-55%. The flame structure, axial temperature, soot concentration and radiation flux were examined and compared with those resulted from a prevalent burner. Also, the optical properties of intermediate soot particles were studied by BOMEM FTIR, IR flame photography and TES-1332A digital luminance meter. The results indicated that natural gas molecules injected into the flame were dominantly pyrolyzed into soot particles. These highly luminous emissive particles enhance the total flame emissivity and radiation. Moreover, considering exhaust emission limits, there was an optimum secondary injection rate (40%), which, besides enhancing the radiation of flame, maintained the emission of CO pollutant and UHC lower than the standard values. Furthermore, in the pyrolysis process absorption of heat from flame reduces maximum flame temperature and consequently NOx emission by 35% in the optimum case. (C) 2017 Elsevier Ltd. All rights reserved.