The refining operation of cellulose fibrous suspensions results in fibrillation, hydration, and shortening of fibers. To characterize the viscous behavior of such a suspension during processing, rheological properties of bleached softwood fibers are studied by considering a semi-industrial disk refiner as a parallel plate rheometer. The rheological behavior of fiber suspension is quantified and modeled using a modified Carreau-Yasuda equation. This model does not show discontinuities and enables the description of the pulp suspension behavior for the entire domain of shear rates (up to 2.1 x 10(6) s(-1)). It points out a Newtonian behavior or a combination of a Newtonian followed by a shear thinning behavior after a limiting shear rate. Moreover, the measured apparent viscosity of the fiber suspension in the gap clearance is about 100 times higher than the viscosity of individualized fibers suspension.