It is well known that the performance and the safety may be hindered because of the clinker formation, when high ash content biomass solid fuel is applied to small scale combustion appliances. This study is aimed for investigation of the clinker formation mechanism of high ash content biomass solid fuel. It gives the suggestion that is very important to development and the improvement of the combustion apparatus to arrange clinker formation mechanism of the biomass solid fuel. In this study, the clinker formation situation by the difference that was in combustion form was classified through the combustion experiment using the commercial combustion apparatus. Furthermore, important knowledge considered to help development and the improvement of the combustion apparatus was provided.