A new approach based on the similitude principle is used to evaluate the effect of fluid viscosity on pump performance. A well validated CFD technique of the entire stage of a mixed flow pump including the secondary flow path is utilized to develop this method. Simulations are carried out at different fluid viscosities representing the pump performance in laminar as well as turbulent flow regimes. The non-dimensional parameters such as head coefficient (Ψ), flow coefficient (ϕ) and rotational Reynolds number (Re) are utilized to characterize the pump performance. The head coefficient data for all viscosities collapsed to two separate lines when represented as a function of with each representing the separate flow regime for pump flow. The exponent "a" (called the Morrison number) is characterized as a function of the rotational Reynolds number and the specific speed of the pump. This procedure has the potential to completely characterize the performance of any pump with any specific speed at any operational speed, however, a larger data base is required to completely populate the Morrison number plot.