Abstract: The effect of fillers on the volume shrinkage of the unsaturated polyester (UP) was studied. And the mechanism of CaCO₃ fillers influence on the volume shrinkage of UP was investigated by SEM，DSC，TG and swelling test. With the increase of the fillers in the CaCO₃/UP composites，the volume shrinkage ratio of the composites is improved at a lower filler amount，and then decreased at higher filler amounts. Compared with the composites obtained with the CaCO₃ fillers untreated with coupling agent，the composites obtained with the CaCO₃ fillers treated with coupling agent had a smaller volume shrinkage ratio. It was found that the CaCO₃ fillers influence the volume shrinkage of the composites by the following ways：in the composites the resin is replaced by fillers and the form of cavity between the interface of the resin and the fillers is beneficial to the decrease in volume shrinkage；the decrease of volatility of styrene by the fillers causes the decrease of the crosslinking density of the composites，resulting in increase of the volume shrinkage. The improvement of interface action between the fillers and the resin，and the decrease of volatility of styrene as a result of coupling agent application result in the decrease of crosslinking density，leading to the decrease of the volume shrinkage. 