This is the first part of the experimental investigation on the feasibility of inverse diffusion flame (IDF) for impingement heating. This part is aimed to identify the favorable IDF structure for impingement heating. Seven IDF structures have been observed altogether. Among them the favorable flame structure for impingement heating is identified to comprise a short base diffusion flame and a long premixed flame torch, separated by a contracted flame neck, which acts as a mixing zone for air and fuel and stabilizes the premixed flame torch. This unique flame structure allows the IDF to possess the advantages of both premixed and diffusion flames, with no flashback, high flame temperature and less or no soot emission. The emission measurement shows it emits low level of nitrogen oxides. Its favorable thermal structure and emission characteristics make it a desirable option for impingement heating. (c) 2007 Elsevier Ltd. All rights reserved.