The main objective of this study was to determine ash transformation and deposition characteristic for three types of straw (corn straw, oat straw, and rice straw) combustion at temperatures between 500 and 1000 degrees C. The collected deposits on the sampling probe were characterized with X-ray diffraction and scanning electron microscopy combined with energy dispersive X-ray analysis. The results indicated that the ash forming processes of straw were influenced by fuel composition and temperature. The quantity of corn straw ash collected from deposition probe was noticeably lower than that of oat straw and rice straw due to different contents of K, S, and Si in fuels. The deposition amounts of corn straw and oat straw followed a linear pattern at temperatures below 800 degrees C, while rice straw followed a nonlinear pattern as a function of temperature. Corn straw was an ideal fuel compared to oat straw and rice straw from the points of deposition amounts and appearance. It also can be found that silicon, calcium, potassium, and sulfur were key points in the forming process of ash deposits.