Urea-SCR system, the selective catalytic reduction using urea as reducting agent is a powerfeul technique to reduce nitrogen oxides(NOX) emitted from diesel engines. However a tank of urea(32.5wt%)-water solution can be frozen in low ambient temperature levels of below -11C. The pi\urpose of this study is to understand freezing and melting phoenomena of the urea-water solution, and its can be applied to get the urea-water solution from frozen it within 5 minutes after cold start. Factors considered were the type of heater and the urea tank shape. From the results, it was found that melting volume of cartridge heater B, during 5 minutes of heating period was 83ml when supplying electric power of 150w. Horizontal , which was put in the narrow bottom space of the tank T1, had fast melting characteristic.