We carried out the accurate in situ Hall-effect measurements, the temperature dependence of electrical resistivity measurements and the first-principles calculations in SnO under high pressure. The results of Hall-effect measurements display the carrier transport behavior of SnO under pressure, which indicates that SnO undergoes a carrier-type inversion around 1.3 GPa and an underlying phase transition at 2-3 GPa. In addition, the temperature dependence of electrical resistivity shows that SnO undergoes a semiconductor-to-metal transition around 5 GPa. The calculated band structures based on the first-principles method illustrate that the indirect band gap of SnO vanishes around 4 GPa. In particular, the results of total and partial density of states indicate that the closure of the indirect fundamental gap is mostly attributed to Sn-5s and 5p states hybridized with O-2p states at the Fermi level.