This paper describes the characteristics and mechanism of fly ash deposited in Shell coal gasification process. The chemical composition, particle size distribution, surface topography and elemental composition of fly ash samples derived from coal A and coal B are studied by X-ray fluorescence spectrometer, JX-2000 microscopic image analyzer and computer controlled scanning electron microscopy (SEM/EDX) respectively. The results show that the differences in ash deposition characteristics result from the differences in surface properties and element composition of fly ash particles generated from different coal feed. The fly ash formation and deposition mechanism are discussed based on physicochemical changes in gasification, particle size distribution and surface composition enriched in Ca, Fe in fly ash particles. The results can serve as reference data for a variety of deposition studies.