Ultrafine phosphorus slag (UFPS) with surface area greater than 6000 cm²/g is a vitreous powder similar to granulated blast-furnace slag (GBFS). The comparative investigation of UFPS and GBFS in this paper showed that UFPS could improve the fluidity, strength, and durability of concrete at partial replacement of cement. With 275 kg/m³ compound UFPS, 27 5 kg/m³ Grade 525^# ordinary portland cement, 0.26 water-binder ratio, and 9.7 kg/m³ high-range water-reducing admixture (HRWRA), a concrete had been prepared with a slump of 240 mm, 28-day compressive strength 101 MPa, and a diffusion coefficient of Cl about one third lower than that of the control portland cement concrete.