The structure and combustion mode of inverse diffusion flame of CH4 and O-2 in hot syngas coflow are numerically studied to gain a fundamental understanding of the flame in noncatalytic partial oxidation (NC-PDX) reformer. The configuration is modified based on the burner system of Cabra et al. [Combust. Flame 2005, 143 (4), 491-506] to make the flame representative of that in NC-PDX reformer. The Eddy Dissipation Concept (EDC) model with the detailed GRI 3.0 mechanism is used to model the turbulence reaction interactions. Results of the study show that the flame is stabilized by autoignition with a wide reaction zone located far away from the stoichiometric line. Analyses on combustion mode show that the flame is established in Moderate and Intense Low-oxygen Dilution (MILD) mode. The inverse diffusion flame configuration which ensures a fully dilution of oxygen plays a key role in achieving MILD combustion in fuel rich coflow. The Increase of coflow temperature or decrease of jet velocity within the range of this study can lead to an early autoignition, but doesn't change the combustion mode. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.