Knowledge of viscosity of flow streams is essential for the design and operation of production facilities, drilling operations and reservoir engineering calculations. The determination of the viscosity of a reservoir fluid at downhole conditions still remains a complex task due to the difficulty of designing a tool capable of measuring accurate rheological information under harsh operational conditions. This dissertation presents the evaluation of the performance of a novel device designed to measure the viscosity of a fluid at downhole conditions. The design investigated in this study addresses several limitations encountered in previous designs. The prototype was calibrated and tested with fluids with viscosities ranging from 1 to 28 cp under temperatures ranging from 100 to 160oF. Viscosity measurements were validated with independent measurements using a Brookfield viscometer. We proposed a mathematical model to describe the performance of the device for Power-law fluids. This model describes the response of the device as a function of the rheology of the fluid and the physical dimensions of the device. Experimental data suggests the validity of the model to predict the response of the device under expected operating conditions. This model can be used to calculate optimal dimensions of the device for customized target applications.