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In a companying paper; a nano-silica (NS) was locally produced from rice husk [1]. This product was used to improve the microstructure and hardened properties of High strength concrete (HSC) containing 15% silica fume (SF). Six mixes were prepared with four different contents of NS of 1, 2, 3 and 4% of cement content by weight and two different contents of cement of 400 and 450 kg/m3, while NS was kept constant at 3%. The hardened properties were measured in terms of compressive strength at ages of 7, 28, 56, 120 and 180 days and splitting tensile strength and young's modulus at 28 days age. The microstructure characteristics were measured using scanning electron microscope (SEM). The experimental results showed that the hardened properties of HSC containing micro-silica (SF) were considerably improved with adding small dosages of NS. In view of hardened properties and cost, the optimum NS content was found at 3%. The SEM study of the microstructure showed that the NS filled up the pores and reduced CaOH2 compound among the hydrates. This mechanism explained the supreme hardened properties of HSC with nano-silica