The fine-scale mixing morphology of vapor explosion and droplet entrapping was visualized by high-speed digital video cameras at an interface between a water droplet and a molten tin pool. The cluster of bubble generated by spontaneous bubble-nucleation covered the whole contact area at 0.1ms after the impingement. Prominent fine mixing between two liquids were found to start at 0.6ms that resulting in vapor explosion. Droplet entrapping phenomenon frequently occurred on an oxide layer, since coherent mixing was prevented due to unevenly formed oxide layer.