The influence of calcium treatment on the mechanical properties of a plain carbon steel (SAE 1050) was investigated. The mechanical properties investigated were tensile and impact strength, fatigue crack growth rate, and the fatigue threshold. Impact testing was conducted at both room temperature and at -40°C. Several heats of both calcium and non-calcium treated steel (SAE 1050) were tested in both the as hot-rolled condition and in the quenched and tempered condition (with a hardness level of HRC = 45).The results of this investigation show no significant difference in the tensile properties or room temperature impact properties between the calcium treated and the non-calcium treated steels. However, the impact strengths of calcium treated steels were slightly higher than that of non-calcium treated steels at -40°C. The fatigue threshold was lower and the near-threshold fatigue crack growth rate was higher for calcium treated material than for the non-calcium treated material, both in the as hot-rolled condition and in the quenched and tempered condition. This difference in fatigue threshold and fatigue crack growth rate has been determined to be caused by the difference in size of inclusions in these two materials.