The compositions of a series of ethylene-propylene copolymers were quantitatively determined by 13C-NMR at 300K in the solvent o-dichlorobenzene. The monomer composition and sequence distribution were analyzed. The glass transition temperature Tg, half temperature of decomposition and crystallinity were found to be closely related to the sequence structures of methylene. The Tg, half temperature of composition, and crystallinity appeared an abrupt change when the methylene length arrived 7~8 because of the movement of the ethylene segments. Also, the Tg of copolymers were simulated according to the monomer compositions. The simulated Tg tended to decrease with the increasing methylene, which was opposite to that of experimented. That was due to the increased crystallization along with the growth of methylene length. While the polymer was amorphous constructed in the simulation, the Tg tended to decrease with the increasing soft segment methylene.