In this research, improving the performance of magnesia-carbon refractories being used in steel making was attended. For this purpose effect of additives and grain size distribution on the quality of magnesia- carbon refractories was investigated. The effect of some additives such as zircon, fused spinel, chromite, carbon black, calcined alumina, aluminum powder, graphite, silicon carbide, boron carbide and mixtures of them were studied. Changing grain size distribution of refractory in accordance with the theory of Anderiasen and the effect on physical, mechanical, oxidation resistance and lag resistance of magnesia - carbon refractories were investigated. Samples tempered at 100, 200 and 250 °C. Then typical work conditions was performed for 5 hours at temperatures below 1650 °C under Reducing conditions. Some samples were placed with steelmaking slag. After curing, the apparent porosity, bulk density, water absorption, oxidation resistance were analyzed. Mechanical strength test was performed according to Brazilian method. After firing the samples were examined usingOptical microscopy, X-ray diffraction and secondary electron microscopy. Results showed that the mechanical strength of samples with additives of fused spinel, carbon black, calcined alumina, aluminum powder, silicon carbide, boron carbide and mixtures of zircon and silicon carbide, improved and mechanical strength of samples containing chromite, graphite, zircon were inappropriate. Calcined alumina, silicon carbide and boron carbide improved the oxidation resistance of refractory. Calcined alumina, silicon carbide, boron carbide and zircon improved the slag resistance of Magnesia - carbon refractory. The results showed that using mixture of calcined alumina, boron carbide, and silicon carbide and small amounts of zircon in the body of Magnesia - carbon had Best performance i mechanical and slag resistance tests. Keywords: magnesia– carbon refractories, additives, mechanical strength, slag resistance, oxidation resistance.