The cesium trapping characteristics with changing reaction temperature, carrier gas and gas velocity by the fly ash filter were analyzed. The SEM (Scanning electron microscope) on the pore structure of the fly ash filter showed that pores up to 0.1 mm in diameter were widely interconnected with each other throughout the whole structure of the filter. According to the XRD (X-ray diffraction) analysis for the cesium compound trapped on the fly ash filter, the thermally stable pollucite phase was formed. The cesium trapping quantity of the fly ash filter was increased with increasing reaction temperature, whereas it was decreased with increasing gas velocity. SEM showed that the fly ash filter after trapping gaseous cesium had mullite phase of needle-like crystals and pollucite phase of bulky crystals with rough surface.