Experiments were conducted to study the effects of H-2, overall equivalence ratio, and airside oxygen concentration on the structure and reaction zone of an LPG circumferential inverse diffusion flame (CIDF) with various hydrogen additions. Planar laser-induced fluorescence (PLIF) technique was used to diagnose distribution and concentration of hydroxyl radical (OH) inside the flame. The result shows that hydrogen addition can significantly improve combustion of the LPG IDF. Although 100%H-2 IDF contains double flame structure, LPG-H-2 IDF seldom shows the structure even when H2 percentage is as high as 90%. Under the low air jet velocity and high overall equivalence ratio condition, LPG IDF shows a large and intense reaction zone, which is similar to that found in a turbulent premixed flame. Oxygen enhancement on the oxidizer side could significantly improve the combustion without changing the overall equivalence ratio. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.