This paper presents a study on the quantitative estimation of bubble size in pressurized fluidized bed combustors (PFBC). A generalized correlation of bubble size which fits the experimental data quite well has been developed for PFBC on the basis of a comprehensive analysis of previous work. The correlation takes into account the different flow regimes at different pressures and gas velocities, as well as the special variation of bubble size within the lower pressure range of the bubbling regime. Using this correlation, an ideal operating area of the PFBC can also be defined from the viewpoint of hydrodynamics and mass transfer. In addition, the correlation can be applied to atmospheric operations to overcome problems encountered by the previous correlations under high gas velocities (e.g. turbulent fluidization regime).