The pearlite formed in 100Mn13 high carbon high manganese steel after 1323 K water toughening subsequently 923 K aging treatment is studied by transmission electron microscope and high-resolution technique. The results show that the pearlite is composed of three phases, which are M3C lamellae, M7C3 lamellae, and ferrite lamellae, respectively, with the orientation relationships of ( 01 1 ¯ )α∥( 2 2 ¯ 1 ¯ )M7C3, ( 1 ¯ 01)α∥( 10 2 ¯ )M3C, ( 2 1 ¯ 1)M3C∥( 2 1 ¯ 3 ¯ )M7C3, [ 251 ]M3C∥[ 542 ]M7C3∥[ 111 ]α. And M7C3 is formed at the phase interface between M3C and α. The growth physical model of three-phase pearlite is proposed. [ABSTRACT FROM AUTHOR]