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A new torsional piezoelectric vibrating resonator was proposed for measuring the viscosity of Newtonian fluids. It consists of a simple and compact structure as a piezoceramic fiber with parallel helical electrodes around outer surface. Principle of the torsional vibration, theory of the fluid coupling, and methods for the viscosity measurement were presented. A prototype of resonator was fabricated and tested in standard viscosity oils with viscosity × density values from 20 to 1500 Pa·s·kg·m-3. Experimental results showed a good agreement with theoretical analysis, especially in high viscosity range. With low material and fabrication cost, miniaturized torsional piezoelectric fiber will provide a competitive way for the portable viscosity measurement or in situ monitoring of viscous liquids, such as lubrication oil and fuel in constrained space.