Abstract LavaSIM is a lava -flow simulator to carry out three-dimensional (3D) analysis of solid–liquid two-phase lava flows. Heat transfer between molten lava and solidified crust into the air, water and ground is calculated using radiation equations, so we can simulate not only the lava-flow distribution but also its physical characteristics: for instance, the internal convectional structure. Lava viscosity can be treated as a function of temperature, and is associated with the percentage of crystallization. The stop condition for the lava flow is determined by calculating the minimum spreading thickness, taking into consideration the yield strength. This paper also discusses whether LavaSIM, the deterministic lava-flow simulation, can be applied to basaltic lava flows and allow lava-flow characteristics, such as inundated area, temperature distribution, crust–melt distribution, velocity and pressure field, to be quantitatively evaluated.