We have numerically demonstrated refractive index sensing using terahertz metamaterials comprised of single split gap resonators. Sensing capabilities of odd and even order resonance modes are precisely investigated. In this scheme the top surface of metamaterials array is covered with a thin layer of polyimide whose refractive could be changed manually. The sensitivity and corresponding figure of merit (FoM) of several lowest order modes are examined with respect to the different thicknesses of the coated polyimide film. We have investigated the electric field distributions at different resonances for the MMs. Although we have mainly focused on refractive index sensing but this study could be extremely useful for the development of metamaterials based sensing devices, bio-sensors etc.