The crystal structure of syndiotactic 1,2-poly(E-3-methyl-1,3-pentadiene) has been determined by the combination of X-ray diffraction and molecular mechanics. Energy minimizations performed by various potential functions yielded models of crystal structure with calculated X-ray powder profiles similar to the experimental profiles. The best model has been refined in order to fit all the experimental data. The space group is Pcam, and the parameters of the unit cell are a = 15.4 angstrom, b = 7.45 angstrom, and c = 5.15 angstrom. Similarities and differences with the crystal structures of syndiotactic 1;2-poly(1,3-butadiene), syndiotactic 3,4-poly(2-methyl-1;3-butadiene), and syndiotactic 1,2-poly(E-1,3-pentadiene) are discussed.