Effect of elastic/plastic mismatch on the fracture of Si3N4 coating in Si3N4/Si3N4 -BN bilayer was investigated by Hertzian indentation testing. A different amount of mismatch between two layers was induced by different BN addition in the substrate layer, and Hertizian cracks were induced by using WC ball indenter. As a result, as the elastic/plastic mismatch between coating and substrate layer increased, the coating fracture easily occurred. A bending stress induced by different elastic/plastic mismatch was main reason to cause the fracture of coating.