This paper presents an experimental investigation of the inert gas effect on flame length. NOx and soot free length fraction (SFLF) in a laminar LPG diffusion flame. Besides this, flame radiant fraction and temperature are also measured to explain observed NOx emission and SFLF. The inert is added to both air and fuel stream at each base line condition by maintaining a constant mass flow rate in each stream. Results indicate that inert addition leads to a significant enhancement in flame length for air-diluted stream than fuel-diluted stream. However, the flame length is observed to reduce with increasing reactant temperature. It is also observed that the SFLF increases with addition of N-2 for fuel-diluted stream. In contrast, SFLF remains almost constant when N-2 is added to air stream. The decrease in fuel concentration and gas temperature caused by inert addition leads to reduction in soot volume fraction and hence enhances SFLF. Interestingly, the SFLF reduces with increasing reactant temperature, due to reduction in induction period of soot formation caused by enhanced flame temperature. Besides this, the reduction in NOx emission level with inert addition is also observed. For all the three cases, the air dilution proved to be much efficient in reducing NOx emission level as compared to fuel dilution. This can be attributed to the differences in reduced gas temperature and residence time between air and fuel-diluted streams. On the contrary, NOx emission level enhances significantly with increasing reactant temperature as a result of increase in thermal NOx through Zeldovich mechanism. (C) 2010 Elsevier Ltd. All rights reserved.