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In this study, fabrication of a composite containing the ordinary Portland cement (OPC) and magnetite (Fe 3 O 4 ) micro/nanoparticles is reported. In the first stage, the cement paste samples with a fixed 0.2 wt.% Fe 3 O 4 additive in four different particle sizes (20-40 nm, 80-100 nm, 250-300 nm, and 1-2 µm) were prepared to check the effect of magnetite size. Magnetite was found to play an effective role in reinforcing cement matrix. The results showed that the cement paste reinforced by magnetite nanoparticles of 20-40 nm size range had the highest compressive, flexural, and tensile strengths compared to those of the other samples reinforced by larger particles. In the second stage, various amounts of the Fe 3 O 4 nanoparticles of 20-40 nm size range were added to the cement to evaluate the influence of magnetite amount and find the optimized reinforcement amount. It was revealed that adding 0.25 wt.% Fe 3 O 4 nanoparticles of 20-40 nm size range, as the optimal specimen, increased the compressive strength, flexural strength and tensile splitting strength by 23-32, 17-25, and 15-19%, respectively, and decreased the electrical resistance by 19-31%.