An investigation was carried out on the effect of sodium sulfate concentration on the sulfate resistance of mortars. Experiments were carried out on the RILEM portland cement standard mortars and portland cement-silica fume mortars. Sulfate exposure of mortars were initiated after 28 days of lime saturated water curing. Some physical and mechanical properties were determined periodically up to 300 days of exposure. Low concentrations of sodium sulfate not exceeding 18000 mg/L had not any significant effect on the compressive and flexural strength of mortars. However, at a concentration of 18000 mg/L some of the properties, i.e. Volume density, volumetric water absorption, indicated beginning of rapid deterioration of mortar structure at an exposure time which could be called critical time. Concentration of 72000 mg/L caused sharp strength reduction between 90 and 180 days for both compressive and flexural strengths. Silica fume replacement caused significant increase in sulfate resistance of mortar even at highest sulfate concentration.