In this paper, we report the extraordinary temperature phenomenon of two-beam coupling in the Ce: Eu: LiNbO3 crystal occuring at about 55, 70 and 110℃. The phenomenon was related to the phase transformation of the crystal structure at these temperatures. An internal electric field is induced by the phase transformation. Then, the structure of space electric charge field is varied, thereby the two-beam coupling is influenced. The formula of phase transformation electrical field (PTEF) changing with temperature is obtained by means of phenomenological method. The theoretical curve in accord with experimental one was given by using the formla and the one-carrier and one-level band-conduction model, which is in accordaace with the experimental curve. The order of the PTEF, 10５Ｖ·ｍ-1，was calculated, which is the value between the order of diffusion electric field and maximum space electric charge field and is the same as the one reported iv Ref. 7.