The objective of this experimental work was to study the rheological behavior of blends composed of unsaturated polyester resin and calcium carbonate. The influence of calcium carbonate content on the viscosity of the resin has been first investigated for calcium carbonate contents ranging from 0 to 60 wt%. Furthermore, the influences of shear rate and standing time have also been analyzed for different resin/fillers blends. The experimental results show a complex rheological behavior, and allow us to better control the composite formulation as a function of the required molding characteristics and properties of the final composite.