Effects of step change of heating sources on the crystallization of zeolite 4A from coal fly ash by hydrothermal reaction were investigated with emphasis on the change in the crystallinity of the synthesized zeolite 4A. Most of the Si and Al components were effectively transformed into zeolite 4A by step change of the first conventional heating and then the second microwave heating of synthesis mixture dissolved from coal fly ash, and maximum crystallinity of zeolite 4A obtained was 91%. The first conventional heating also plays an important role in enhancing the nuclei formation that Si and Al in synthesis mixture reacted to form ring-like structures for combining sodalites, and further to small zeolite 4A seeds. The secondmicrowave heating increases the crystallization rates from small zeolite 4A nuclei to more zeolite 4A crystals. The cation exchange capacity (CEC) of the zeolite 4A crystallized by step change of heating source from the conventional to the microwave was 5.5 meq/g compared to 5.7 meq/g for commercial zeolite 4A. Test results showed that removal efficiency of heavy metals by zeolite 4A synthesized from fly ash was more than 98% and similar to commercial zeolite 4A.