A binary-medium model is proposed for over-consolidated clays within the framework of breakage mechanics which has been advanced recently by the first author. In this model the heterogeneity inherent in the soils is taken into account; and a soil is regarded as a composite material consisting of structural lumps and structural bands to resist the external loads commonly. The model has 10 paramenters; and a method of their determination is also proposed. The results obtained from simulating triaxial test show that the model can reflect the mian features of the behavior of London clay.