The electrical resistivity ρ of Mo films evaporated by an electron‐beam gun onto SiO2/Si substrates has been studied in relation to the film structure and purity. The resistivity was profoundly influenced by the substrate temperature during deposition T s but was almost independent of film thickness d if d exceeded 1000 Å. When the films were heat treated at above 700°C, ρ decreased as the grain size increased. These results can be explained if one assumes that the resistivity is due to grain‐boundary scattering. The reflection coeficient of the electrons at grain boundaries R depends on T s , but shows no change with heat treatments at temperatures up to about 1000°C(R =0.41/0.21 for T s =300/600°C). It is shown that R is proportional to oxygen content in the grain boundaries.