The mechanism of crack propagation in the interphase plays an important role in the failure procedure of ceramic matrix composites (CMCs). The aim of this paper is to examine the effect of interphase thickness on the propagation of a pre-defined matrix crack. A SiC/SiC unit-cell with different thicknesses of BN interphase was analyzed using an approach based on the extended finite element method (XFEM). With this approach, the initiation of new cracks and propagation of primary crack in the interphase can be simulated without the presupposed crack path. The crack patterns and stress-strain responses of unit-cells with different thickness were compared. The initiation and propagation of the secondary crack/cracks in the interphase provide insight into the toughening mechanism and design of interfaces/interphases of CMCs. [GRAPHICS] .