InP MISFETs with Al//2O//3/native oxide double-layer gate insulators grown by simple anodization processes in electrolyte and in oxygen plasma have been investigated. Device performance was greatly improved by adding thin native oxide intermediate layer, showing effective electron mobilities of 1500-3000 cm**2/V. s with marked reduction of the current instability. A mechanism of the current drift was proposed, concerning to usual interface states below semiconductor conduction band edge. Feasibility of the double-layer gate MISFET ICs was also demonstrated by E/D inverters and ring oscillators.