Physical properties of cement mortars modified by hydroxyethyl methyl cellulose(HEMC), ground granulated blast furnace slag and superplasticizer, such as air void content, water retention and bulk density of fresh mortars, as well as bulk density, porosity and capillary water adsorption of hardened mortars, were studied. The results show that effects of superplasticizer and ground granulated blast furnace slag on water retention and capillary water adsorption of mortar are similar to those of hydroxyethyl methyl cellulose. However, effects of the former on air void content, bulk density and porosity are opposite from those of the later. And hydroxyethyl methyl cellulose counteracts part effects of superplasticizer and ground granulated blast furnace slag, which results in bulk density and porosity of mortar maintaining in certain value.