The effect of the amount of calcium carbonate (CaCO3) on water absorption of the cured unsaturated polyester resin (UP)-CaCO3 system in boiling water has been studied and the correlations between the retentions of flexural strength and modulus and the water absorption of the system were discussed.When the samples were soaked in boiling water, the water absorption decreased with an increase of CaCO3 content at first, became maximum at a certain CaCO3 content and then decreased with an increase of CaCO3 content. This phenomenon suggests the two effects of CaCO3 that the incorporation of CaCO3 lowers the water absorption at a short period of soaking but raises the water absorption at a long period of soaking.Both of the retentions of flexural strength and flexural modulus decreased with an increase in the water absorption, but the incorporation of CaCO3 reduced such decreasing tendency of the retention of flexural strength, while it promoted that of the retention of flexural modulus.From these results, it is proposed that the decrease in UP content resulting from the increase in CaCO3 in the system causes the reduction of water absorption by UP and suppresses the decrease in retention of flexural strength, while the increase in interfacial area by the increase in CaCO3 content promotes water absorption at the interface and, thus, accelerates the decrease in the retention of flexural modulus.