An approach was developed for observation of the wetting behavior of molten magnesium on solid substrates by pre-filling magnesium vapor in the reaction chamber from a special magnesium evaporation source to control its evaporation from the sessile drop. Based on this approach, the wetting behavior of four kinds of substrates (alumina, iron, SUS316 and SUS430 stainless steel) by molten magnesium was investigated. The results showed that the molten magnesium drop revealed a spreading tendency during the initial wetting process stages on alumina and SUS316 stainless steel solid substrate and gave a good wettability of 89° and 71°, respectively, for the initial contact angle at 973 K. In these wetting processes, there were two distinct behaviors of the advancing wetting because of the good wettability of molten magnesium and the subsequent drop-receding, resulting from the evaporation of magnesium. Furthermore, the interfacial reaction products that induce good wettablity could be observed in other specimens obtained under higher reactive conditions, although they were hardly detected at the interface between molten magnesium and the substrate at the end of the wetting experiment. On the contrary, the molten magnesium drop did not advance on the surface of iron and SUS430 substrates because of the poor wettability at 115° and 116°, respectively, for the initial contact angle at 973 K.