The sticker emission standards (Euro 5, 6, and 6C) for commercial diesel vehicles in the European Union have been issued for last two decades. These growing demands on NOx emission reductions require the need of better after treatment methods in the automotive industry. One of the crucial technologies for NOx reduction is by after treatment of the exhausts with a Urea SCR-system (i.e., the selective catalytic reduction using urea as a reducing agent). In Urea SCR-system a mixture of urea and deionized water (urea water solution, UWS, 32.5wt%) injects into a catalytic reactor in which urea converts NOx into nitrogen gas and water. In heavy duty diesel vehicles, the solution in the UWS tank should be supplied to the injection system in the liquid phase. In the present work, flow and heat transfer characteristics in the UWS tank for heavy duty vehicles are investigated numerically. The conservation equations for mass, momentum, and energy including the heat conduction and natural convection with phase change processes are solved by the STAR-CCM+, which is one of the commercial S/W package. The enthalpy porosity method is adopted to obtain the heat transfer characteristics included the melting amount of UWS. In addition to, the effects of shape of tank cover and supplied heat on the melting amount are also investigated.