© 2018 The Japan Society of Applied Physics. An optical mode converter between hybrid device and Si waveguide is the key component for efficient and stable operation of III-V/silicon-on-insulator (SOI) hybrid photonic integrated circuits (PICs). In this study, we introduced a double taper structure into such a mode converter and investigated the coupling efficiency dependence on their structural parameters. By using N2 plasma activated bonding technology, III-V/SOI double-taper-type mode couplers with various taper-tip-widths and taper lengths were fabricated, and their coupling efficiencies were evaluated. As the result, a coupling efficiency of as high as %0.2 dB was achieved for a double-taper device with a tip width of 0.4 µm and total length of 85 µm.