An innovative sensing system is proposed, designed, and fabricated for very viscous fluid (engine oils) viscosity measurement. The proposed sensing system is based on a torsional resonator, which is composed of a hollow shaft, a macro fiber composite (MFC) and a PZT transducer. To measure the viscosity, the resonator is fixed at one end and partially immersed into the viscous fluid. The resonator is excited by the MFC and the tip vibration of the torsional resonator is monitored by a PZT transducer. The mathematical modeling is derived based on the hollow shaft and the interaction between the resonator and viscous fluid is modeled by the Stokes’ Theory. Different from traditional vibration-based sensing systems that employ frequency shift to evaluate the viscosity. The proposed sensing system investigates the resonator’s peak-peak amplitude at resonance, which is proved to be more sensitive to very viscous fluid viscosity change. FEA simulation is conducted by COMSOL to verify the mode shape and the frequency response with different viscosities. Lab experiment are carried out to verify the theory and the FEA simulation Outputs are normalized for calibration and the results show a good matching among the theory, FEA simulation and experiment. The torsional resonator is effective for very viscous fluid viscosity measurement, such as engine oil, concrete mixture, and other fluids that viscosity varies over time. The proposed sensing system can be applied to automobile manufacture, food industry as well as structural engineering.