The effects of Zr on the microstructures and mechanical properties of microalloyed steels have been investigated by mechanical tests and microstructural observations. The microstructures in the Zr-doped steels are ferrite plus pearlite, which is similar to those in the Zr-free steel. With the increase in the Zr content, the lamellar structure reduces and even disappears. Sulfides and silicates that exist in the Zr-free steel are modified into fine oxides in the Zr-bearing steel. When the Zr contents range from 0.01wt% to 0.03wt%, the low temperature toughness of the steel can be substantially improved while its room-temperature strength and ductility have no apparent change. The refinement of ferrite grain size by the addition of zirconium is one of the main reasons for this toughness improvement. The effects of Zr on the microstructures and mechanical properties of microalloyed steels have been investigated by mechanical tests and microstructural observations. The microstructures in the Zr-doped steels are ferrite plus pearlite, which is similar to those in the Zr-free steel. With the increase in the Zr content, the lamellar structure reduces and even disappears. Sulfides and silicates that exist in the Zr-free steel are modified into fine oxides in the Zr-bearing steel. When the Zr contents range from 0.01 wt% to 0.03 wt% , the low temperature toughness of the steel can substantially substantially while its room-temperature strength and ductility have no apparent change. The refinement of ferrite grain size by the addition of zirconium is one of the main reasons for this toughness improvement.