Growing interface structure of pearlite in 100Mn13 high carbon high manganese steel was studied after 1323 K solution treatment and subsequently 823 K aging treatment by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The results show that longer pearlite lamellae consisting of ferrite lamella and cementite lamella are formed in initial growth stage of pearlite colony, in which there are protrusions and branches in M3C/γ interface. The cementite lamella is divided along (01¯0) M3C into two parts which differ in some respects, such as edge dislocations, (01¯0) M3C lattice arrangement. Besides, edge interface of ferrite lamella is wavy morphology including two peaks presenting small curvature radius, which accommodate more C atoms under the premise of meeting composition condition for transformation to accelerate the transformation. The growth physical model of cementite lamella with protrusions and branches, ferrite lamella with two peaks and three troughs are respectively proposed.