Solid fuel combustion is always preceded by chemical decomposition process. The performances of decomposition process are greatly affected by geometry and operating conditions. This study is aimed to evaluate influence of axial inlet geometry on the decomposition process performances in a burner. The decomposition process performance was evaluated through fluid dynamics characteristics, such as backflows pattern, mixing intensity and heat transfer rate. The fluid dynamics characteristics were evaluated by standard k- turbulent model, using Ansys-Fluent software. From simulation results, it was observed that axial inlet diameter (Dai) significantly influenced fluid dynamics characteristics in the burner. Of all axial inlet diameters used, there was vortex from furnace to burner cylinder. This vortex had potential to increase the decomposition performance by keeping the burner temperature high. The structure of vortex was determined by the Dai. The mixing intensity and heat transfer rate, which can also be used as decomposition performance indicator, were also dependent on Dai. The best decomposition performance was generated at Dai = 0.1 m or 33% of burner cylinder diameter. Under this condition, vortex penetration reached 82% of the burner cylinder length and turbulent intensity as well as heat transfer rate were relatively high.