Computational study has been performed to observe the flow characteristics of combustion chamber for geometrical modification in municipal solid waste incinerator. A series of geometrical modification has been carried out as an attempt to reduce the size of recirculation zone, to obtain uniform flow field in the secondary combustion chamber and to improve the mixing of combustion gas. Two dimensional non-reacting turbulent flow has been studied as the first step to get such goals and the result of design optimization is presented. In addition, three dimensional non-reacting and reacting flow analyses were performed to verify the validity of two dimensional approach.