Circulating fluidized bed (CFB) combustion techniques have been widely used in China. In order to improve CFB boiler performance it is necessary to study on microstructures of fly-ash. With the help of nitrogen adsorption instrument and scanning electron microscopy, the pore structure of fly-ash in circulating fluidized bed boilers are studied by nitrogen adsorption/desorption isotherms of fly-ash, hysteresis loop and pore distribution. The results indicated that different particle sizes of fly-ash in CFB boilers are of similar nitrogen adsorption isotherms, pore types and the pore size distribution, and the most probable pore radius of fly-ash is about 2nm. Adsorption isotherms of fly-ash is the second type, and the macro-porous and meso-porous types are tapered hole, parallel plate slit hole and the ink bottle shape hole. And meso-pore in proportion is the largest. Micro-pores are not discovered in fly-ash from CFB boilers.