Based on a conventional torsion pendulum,we develop a forced oscillation viscometer with ultra-high viscosity sensitivity of 2 × 10-7 Pa.s working at frequencies near the resonance.The viscosity is achieved by exploiting the phase lag for the angle displacement behind the torque,instead of the resonant curve,i.e.,the variation of angle displacement amplitude versus frequency.The general formula for the measurement of the visco-elasticity of complex fluids is also presented.With such precision it is easy to measure tiny change in viscosity result from circumstantial influences.Deionized water and two kinds of NaCl aqueous solutions are chosen to demonstrate the performance of our home-made torsion pendulum-based viscometer.