Thermal expansion, expansion by water sorption, water sorption and solubility to water, which will influence to the marginal closure of cavities restored, were evaluated in seven microfilled composite resins, comparing with two conventional composite resins. The following results were obtained. 1) Coefficients of linear thermal expansion of microfilled composite resins were 41 to 84 x 10(-6)/degrees C in the dry condition and 57 to 93 x 10(-6)/degrees C in the wet condition. Those values were about 2 to 3 times those of conventional composite resin, and about 6 to 8 times that of teeth. 2) Linear expansion by water sorption was highly correlated to water sorption. Microfilled composite resins had higher values of liner expansion by water sorption compared to conventional ones. 3) Generally, water sorption of microfilled composite resins were higher than those of conventional ones. After 7 days, they were about 0.4 to 1.9 mg/cm2. Values of microfilled ones which contained larger amount of filler or was composed of more hydrophobic matrix resin, however, were lower than others. 4) Solubilities to water of microfilled composite resins also were higher than those of conventional ones. Particularly, microfilled ones which were used organic composite filler and UDMA monomer as base material and photopolymerized had relatively higher values.