This paper presents a numerical simulation of the interaction between smoke and water mist in a fire using CFD software package Fluent. Motion of droplets of different size in the flow field is studied and their fire extinguishing ability is compared according to whether they can reach the combustion zone. Scattering of thermal radiation from fire by water mist and the effects of breakup and coalescence of droplets at initial stage are also studied. Modeling results demonstrate that radiation is the main heat transfer process;bigger droplets are easier than smaller one to get away from the circumfluence zone and to arrive at the combustion zone. Water mist attenuates thermal radiation by absorbing and scattering, and it may enhance combustion reaction rate at initial stage. These results agree with experimental result.