Wettability of SiO2, PG and PBN substrates by molten Tin were measured by Sessile drop method under hydrogen atomosphere after evecuation by vacuum pumps. To avoid the surface contamination due to organic absorption originated from vacuum systems, a baked vacuum vessel, a diaphragm pump and a turbo molecular pump were employed. SiO2 and PBN substrates were intentionally contaminated in vacuum using a rotary pump and a oil diffusion pump to evaluate the effect of contamination on wettabilities. For SiO2, the contact angles on cleaned and contaminated substrates were 124 and 136 degrees, respectively. For PBN, they were 158 and 154 degrees, respectively. The contact angles on contaminated substrates were closed to 141 degrees on cleanes PG substrate, suggesting that the surface becomes more similar to that of PG thorough vacuum treatment. Surface tension of molten tin was affected by neither materials nor contamination of substrates and it can be expressed by an equation: gamma = 571.2-0.071T±5.0 mN/m where T is the temperature in Centigrade. Works of adhesion for wetting of cleaned SiO2, PG and PBN were 253, 124 and 40 mJ/m, respectively. However, for wetting of contaminated SiO2 and PBN, they were decreased to 156 and increased to 56 mJ/m, respectively.