Unsaturated polyester (UP)/organically modified clay (OMC) nanocomposites were prepared by multistep simultaneous mixing of UP oligomer chain, styrene (St) monomer, and OMC. X-ray diffraction, transmission electron microscopy, dynamic mechanical thermal analysis, and scanning electron microscopy data were in support of the formation of a partially intercalated nanocomposite. The glass transition temperatures of the nanocomposites revealed that the crosslinking reaction occurred homogeneously inside and outside of the OMC galleries. Adding 3 wt% OMC improved the flextural and storage modulus of UP by 31.5% and 30.2%, respectively. The Izod impact strength of UP was also improved by 51.7% at 1 wt% of OMC loading. Similar results were obtained for low-profile UP/St/OMC nanocomposites. Resin shrinkage data measured by inhouse constructed apparatus showed that, at an OMC content of 3 wt%, the UP/St/OMC/low profile additive (LPA) system cannot provide superior volume shrinkage control. But, it is found that the use of nanoscale reinforcement in the UP systems is able to restore flextural and storage moduli loss when using LPAs. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers