Abstract: Using boundary element method, a study is given on the inter facial fracture toughness of a bimaterial fourpoint bending specimen with either perfector imperfect interface. The complex stress intensity factor and the strain energy density in the tip of interface cracks are calculated for these two kinds of specimens respectively. It is found that within most distance spanned by the innerloading points the above two quantities at tain essentially constant values. This stable behavior will facilitate the test of interfacial fracture toughness.