In this study, the effect of Cr and Co on microstructure and mechanical properties of low Cr-W steels was investigated. For this goal, 3% Cr-3%W and 5% Cr-3%W steels, this may be an alternative to new generation low Cr steels, containing 0%, 0.5%, 1.5%, 3%, 4.5% Co, produced by casting and hot rolling. The samples were annealed at 1100 °C for 1 h, followed by quenching in air and tempering for 2 h at 710 °C. Ferrite, bainite, and martensite phases were determined in various volume fractions according to the chemical composition. Carbides precipitated within the grain and grain boundaries during tempering. Co bearing caused an increase of A1, A3, and Curie temperatures. Co slightly increased hardness and strength in both 3 Cr and 5 Cr alloys. However, the hardness and strength of 5 Cr alloys with all Co ratios were almost same that of 3 Cr alloys. The Charpy-V impact energies of 5 Cr steel are lower than 3 Cr steel with the same Co amount. The addition of Co up to 1.5% in steels with 3 Cr did not make any major changes in the notch toughness. However, the addition of 3% Co to the alloy significantly reduced impact energy. In 5 Cr steel, notch toughness generally decreased with the increase of Co amount.