The effect of nanoclay on structural relaxation of unsaturated polyester (UP) resin containing 3 wt% Cloisite 10A (UP/10A), 3 wt% Cloisite 30B (UP/30B), and the neat resin was investigated by differential scanning calorimetry (DSC) and temperature modulated differential scanning calorimetry (TMDSC). X-ray diffraction and transmission electron microscopy were used to evaluate the morphology of UP/10A and UP/30B nanocomposites. According to the strong-fragile concept proposed by Angel, the values of the fragility index m were determined. The average size of a cooperative rearranging region (CRR) at the glass transition temperature (T-g) was then calculated. Considering the significant increase in T (the mean temperature fluctuation) of UP/10A, it was concluded that UP/10A system was more heterogeneous than the neat UP and UP/30B specimen. Significant drop in T-g accompanied by an increase in T and a decrease in characteristic length of cooperativity at glass transition temperature ((Tg)) might be a sign of existence of smaller average CRR sizes in UP/10A nanocomposites. Results showed that the increase in the concentration of styrene between galleries of Cloisite 10A caused a decrease in styrene content outside the nanoclay layers leading to diminishing of crosslink density and as a consequence the decrease of (Tg). POLYM. ENG. SCI., 54:2859-2865, 2014. (c) 2014 Society of Plastics Engineers