The rheological behavior of styrene-free unsaturated polyester resin–nanoclay (Cloisite 15A and 30B) nanocomposites at various nanoclay contents prepared by melt mixing was investigated. To investigate the effect of shear and diffusion induced phenomena as well as nanoclay surface modification on the rheological behavior, samples were prepared at two different temperatures of 40°C (cold mixed) and 150°C (hot mixed) and tested in a range of 40–120°C using dynamic rheometry. Viscosity–temperature curves for the cold mixed samples showed a decrease in viscosity with increasing temperature followed by the formation of a plateau with onset temperature which depends on the nanoclay type and content. The results of small angle X-ray scattering and transmission electron microscopy analysis were in agreement with the formation of a physical network in which nanoclay particles act as the nodes, whereas polymer chains serve as the links. It is believed that for the cold-mixed samples, higher shear forces break the nanoclay stacks associations efficiently, and hence more nanoparticles are available for polymer chains to form the network. In this context, the initial d-spacing was much more effective than the nanoclay modification type. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.