The aim of the present study is to investigate the use of diatomite as a partial replacement for cement in the production of cement mortar. Diatomite was used at 0%, 5%, 10% and 15% replacement by weight for cement while sand and water quantities were kept constant. Compressive and flexural strength, freeze–thaw resistance, sulfate resistance, water absorption and dry unit weight of the mortars were determined. The compressive and flexural strength decreased with increasing diatomite content for all curing periods. However the compressive strength of the cement mortar which was produced with 5% diatomite content complied with the minimum specified value of given in the standards. Diatomite replacement generally increased the compressive strength of the cement mortar after 25 freezing and thawing cycles. Water absorption of the mortars decreased with the increase of diatomite content except the mortar containing of 15% diatomite. Dry unit weight of the cement mortar was lower than the control mortar because of high porosity of diatomite. The expansion of the cement mortar bars immersed in 5% sodium sulfate solution decreased with increasing diatomite content and generally the sulfate resistance of the mortars was higher than that of the control mortar.