The effect of tempering temperatures on microstructures and mechanical properties was studied in a low carbon steel. The disappearance of continuous yielding and the formation of an extended region in engineering stress-strain curves at tempering temperatures ranging from 673 to 873K was caused by the reduction of mobile dislocations during tempering and dynamic recovery during tensile deformation. In addition, the occurrence of discontinuous yielding in the sample treated at the tempering temperatures above 923K was attributed to the formation of new strain-free polygonal ferrite grain.