The spheroidization of cold rolled lamellar pearlite in annealing at the temperatures bwtween 600 and 700℃ has been studied by quantitative micrography. It was foud that the spheroidization proceeded as two stageh. The first stage was the stage of relieving the stored energy by cold work, the second was the stage of reducing the interface energy between ferrite and cementite. The spheroidization rate combining the spheroidization rate of each stages is described by the following equation : d(1/S)/dt=k₃·D/_(1/s){σV/_(1/s)+k₄·exp(-bt)} Where, S is the total area of the interface between ferrite and cementite per unit volume, D is the diffusion coefficient, Q is the boundary energy, V is the volume fraction of the cementite, and k₃, k₄, b are constants.