Clay changes the strain-induced crystallization behavior of natural rubber and induces a dual crystallization mechanism due to the orientation of clay layers during deformation. The structure evolution was probed by in-situ synchrotron wide-angle X-ray diffraction, while the thermodynamics of the onset of crystallization was analyzed by the tube model. The entropy change required for the onset of the strain-induced crystallization of the clay filled rubber is composed of the entropy reduction due to the rubber filler interactions and also the stretching. The summation of the two kinds of the entropy reduction is nearly equal to that of the neat rubber. The thermodynamic analysis reveals that the orientation of clay layers along the direction of stretching reduces the chain conformational entropy and changes the crystallization mechanism. These results give some new insights into the strain-induced crystallization process and the reinforcement mechanism for the clay filled rubber. (C) 2011 Elsevier Ltd. All rights reserved.