A Mach-Zehnder interferometer (MZI)-based liquid refractive index sensor, utilizing a hollow hybrid plasmonic (HP) waveguide as the sensing element, has been investigated, showing large sensitivity to the refractive index changes of the tested liquids, as well as lower propagation loss in comparison to typical plasmonic waveguide-based ones. The sensor is fabricated using conventional silicon-on-insulator (SOI) technology; therefore, it is compatible to other standard SOI devices. The waveguide sensitivity, Sw, is experimentally demonstrated to be 0.64, with a propagation loss less than 0.25 dB/μm. Using a 20 μm long hollow HP waveguide in the sensing arm, the sensitivity of the MZI sensor (device sensitivity, Sd) is about 160 nm/RIU, with an extinction ratio larger than 40 dB.