In order to find the optimum atomizing condition for combustion efficiency, a combustion evaluation concept is introduced on the basis of the atomizing characteristics, and the dimensionless parameter Stk (Stokes Number) is used to analyze the trajectories of CWM spray droplets. Based on the these concepts, the Combustion Index CI is defined. The larger the CI values, the better the combustion. It is also found that the atomizing condition at a Wa/Wl of 0.26 is optimum for combustion efficiency. When Wa/Wl>0.26 blow-off occurs due to the increased injecting speed. On the other hand, the burn-out times become much longer because of worse atomization (in the condition of Wa/Wl<0.26). This is a very useful prediction method to evaluate the performance of an atomizer and the combustibility of CWM with various properties. The prediction results above are verified in a small-scale test furnace.