Introduction ============ Nays3DV is 3Dimensional model developed for calculation of vertical and horizontal movement of fluid with density currents. The Nays3DV solver is developed by Professor Yasuyuki Shimizu from Hokkaido University. Density currents occured due to density differences, arise from temperature variations, suspended solids or dissolved materials. Therefore, formation and evolution of density currents are induced by natural conditions such as saline intrusions, oil spills etc. Density flow is important for problems in lakes, reservoirs and estuaries. This manual explains the step by step guide of several examples in Nays3DV. Explanations of this manual are based on the assumption that you have already installed iRIC in your computer. If you have not installed the iRIC software, download it from the following URL and install it on your computer. URL: http://i-ric.org/downloads Software: iRIC version 3.x or later