The adhesion behavior of ash generated during coal gasification under high temperature condition was investigated by using horizontal-type pulverized coal reactor with water-cooled tube. The size distribution and chemical composition of attacking and deposited particles on tube surface were measured by CCSEM (Computer Controlled Scanning Electron Microscopy). The fraction of fine size and low fusion temperature in collision particles selectively adhered to tube surface. The ash adhesion probability to dry surface was determined by the collision efficiency and viscosity prediction of each particle. Ash adhesion model that will be applied to the gasifier was proposed.