Using transmission electron microscope and heating stage techniques, the authors have studied the phase transition of P21ca enstatite(En56Fs41Wo3).When the sample is heated to 900℃ or so, the diffraction spots of k≠2n in OkL diffraction of P21ca enstatite disappear, forming orthopyroxene with Pbca symmetry, The phase transition from P21ca orthopyroxene to Pbca orthopyroxene is achieved through the change in the rotation type of Si-O tetrahedra chain which is Srotation in the structure of P2lca orthopyroxene, This phase transition experimeat demonstrates that P21ca orthop3rroxene is the low-temperature polymorph of Pbca orthopyroxene.