In this paper, we intensively investigate the influence of steam on the hetero-homogeneous joint ignition of dispersed coal particle streams in both O-2/N-2 and O-2/CO2 conditions of an optically accessible flat-flame burner. Two differently ranked coals and one bituminous char were tested for a comparison. The effects of moisture content in the raw coal, steam content or diluents (N-2/CO2) in the ambience, and fuel types were systematically examined under isothermal conditions. First, it is seen that the moisture content in raw coal delays its ignition because of endothermic vaporization, which becomes weakened at the high ambient temperature. Secondly, three main mechanisms are categorized to explore the profound effects of ambient steam content on ignition of coal particles, i.e., (i) the promotion effect of OH radical on the oxidation of volatile hydrocarbons, (ii) the inhibition effect of gaseous H2O molecule on the equilibrium of hydrocarbon oxidation, (iii) the negative effect of endothermic surface gasification reaction (C-CO2/C-H2O). In O-2/N-2 ambience (1500 or 1800 K), as the steam content increases, the ignition delay of coal particles is rapidly reduced in an early stage of 0%-5% due to the positive effect of OH radicals, followed by an inappreciable reduction in 5%-13% stage because of both the steam inhibition effect and the endothermic surface gasification. However, in O-2/CO2 ambience with sufficiently high steam content (5%-18%), the ignition delay even becomes longer with the further increment of steam. It is because of the synergistic effect between C-H2O and C-CO2 reaction at high temperatures (1500 K and 1800 K). Finally, for fuel types, the OH radical promotion effect of two lignite samples is obviously higher than that of low-volatile char samples, whereas with steam addition the sodium-rich Zhundong coal exhibits lower ignition delay than Hulunbel lignite due to catalytic effect.