This article describes the preparation and properties of unsaturated polyester (UP)/montmorillonite (MMT) nanocomposite by UV radiation. Benzildimethyl-ketal and 1-hydroxy cyclohexyl phenyl ketone (HCPK) as a photoinitiator in UP were photolyzed under nitrogen condition. Analysis of X-ray diffraction patterns of the composites shows that the interlayer spacing of MMT is substantially increased. The type and amount of photoinitiator affects the level of improvement in mechanical properties, and our results suggest that HCPK is a more efficient photoinitiator of UP curing reaction. The interaction between photoinitiators and MMT should be considered to acquire the reinforcement effect of the dispersed MMT nanolayers. The properties of nanocomposite are dependent upon the amount of MMT. The effect of MMT concentration on thermal and dielectric properties is also investigated. (c) 2006 Wiley Periodicals, Inc.