An investigation into the cutting performance of newly developed free-cutting steels was conducted at high cutting speeds (150, 200 and 300 m/min) using cemented carbide cutting tools. The tool life, cutting force and surface roughness were measured to study the effect of the Mn/S ratio in weight percent on the cutting performance of the steel. The experimental results confirmed that the Mn/S ratio had a great influence on the machinability of the steel. It was more beneficial to form a MnS lubricant zone during the machining when the ratio is 3.33. The MnS lubricant zone formed on the rake face could reduce the tool wear and improve the tool life. A lower main cutting force could reduce the friction and bonding between the tool rake face and chip. The best machinability of the steel in terms of the tool life and main cutting force was obtained when the ratio was 3.33. The surface roughness decreased as the cutting speed increased when the Mn/S ratio was less than 3.33, and it increased when the ratio was greater than 3.33. The surface roughness of the steel also had a notable connection with the radial force. A properly high radial force could be beneficial in reducing the surface roughness because of the extrusion between the tool and the machined surface.