The electrical properties of n-type InSb metal-insulator-semiconductor (MIS) structures have been investigated. As gate insulators, SiInxPyOz layers are formed on InSb by a new method. In the method, an In-SiOx′ double layer is formed on the InSb (111)B surface by evaporation of SiO and In. Further, a Py′Oz′ layer is deposited on the double layer. The reaction between a Py′ Oz′ layer and In-SiOx′ double layer yields a SiInxPyOz layer. The SiInxPyOz layers exhibit high resistivities ρ, i.e., ρ≊6.5× 1015 Ω cm at 100 K and 1.0×1014 Ω cm at room temperature. The hysteresis in the capacitance-voltage (C-V) and conductance-voltage (G-V)curves of InSb MIS structures at 100 K is found to be less than 0.6 V and an injection type. The surface-state density in the MIS interface is evaluated by including the effects of the nonparabolicity of the conduction band and Fermi–Dirac statistics for electrons in InSb. A typical value of the minimum surface state density is observed as 8.3× 1011 cm−2 eV−1 at about 0.05 eV below the conduction-band edge. Further, the C-V characteristics of InSb MIS structures at room temperature are discussed by including the features of the narrow band-gap semiconductor.