Abstract: Starting from dielectric tensor of magnetic optical materials,this paper analyzed the relation between magnetic circular dichroism(MCD) and Faraday extinction ratio,and showe that the extinction ratio of magnetic optical materials was defined by MCD and MCB(magnetic circular birefraction).As an example,the BiCaInVIG material was used to be a optical isolator to discuss the isolation level and insertion loss.The numerical calculation indicated,if MCD was less than 0.2,the optical isolation level would be less than 30dB.