The successive stages of vapor explosion were video-framed with an exposure time of 500 ns. In order to attain good repeatability and visibility, a smooth round water droplet was impinged onto a molten alloy surface. This configuration suppresses pre-mixing process prior to triggering of vapor explosion. The cluster of bubble generated by spontaneous bubble-nucleation covered the whole contact area at 0.1 ms after the impingement. Prominent fine mixing between two liquids were found to start at 0.6 ms 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.