Investigation of the structure of co-flow methane-oxygen diffusion flame is presented. A numerical model with a 43-steps chemical kinetics mechanism and an optically thin radiation sub-model is employed in simulations. Through simple co-flow diffusion flame experimental arrangement, visible flame extents have been measured. The methane oxygen flames have four distinct visible regions; a dark inner zone, a reddish zone, a highly luminous zone and a blue or bluish-white zone. The variation of the extents of these zones with increasing oxygen flow rate is discussed. The effect of oxygen flow rate on the species and temperature distribution is studied in detail. © 2011 Inderscience Enterprises Ltd.