Coal fly ash was dissolved into NaOH aqueous solution and the mixture of rice husk ash as a silica source with the solution separated from the insoluble fly ash was treated hydrothermally at 373 K with microwave heating and conventional heating. Through this experiment, we investigated effects of microwave irradiation on the crystalline phase of zeolites synthesized from fly ash and clarified what process is intensively affected by the microwave irradiation. As a result, it was found that the product powder synthesized with microwave heating has a relatively higher hydroxysodalite phase than that with conventional heating, even if the synthesis conditions were the same. It was also clarified that promotion of hydroxysodalite generation by microwave irradiation is not induced by the enhancement of the crystalline phase transition from phillipsite to hydroxysodalite, but by the acceleration of hydroxysodalite formation from the precursor, aluminosilicate gel, directly. Moreover, it was found that promotion of hydroxysodalite formation is not due to the heat spot on the surface of the carbon caused by microwave irradiation. We revealed that the sensitivity, which is defined as the ratio of the quantity of generated heat to the energy of the microwave generated by the magnetron, of hydroxysodalite is almost 10 times as high as that of phillipsite. From these results, the following enhancement mechanism of hydroxysodalite crystal generation by microwave irradiation was proposed. When zeolite nuclei grow to the zeolite crystal on the interface between zeolite nuclei and aluminosilicate gel, the crystal growth of hydroxysodalite is promoted selectively as the surface temperature of hydroxysodalite is much higher than that of phillipsite in the case of microwave heating.