Abstract: Natural soft clays are generally subject to the effects of a soil structure. How to reflect the effects in the corresponding constitutive model is an important issue. Based on the modified cam clay model, a parameter designated as structure yield stress was introduced to describe the shape of an initial yield surface due to the soil structure, and an anisotropic parameter was presented to characterize the rotation of a yield surface due to initial anisotropy. According to the mechanism of abrupt loss of a soil structure for natural clays, the elastic stress strain relationship was applied in the pre-yield state, and corresponding hardening law and associated flow rule consistent with a modified cam-clay model were adopted since the resistance of a soil structure in the post-yield state is completely exhausted. According to the condition of consistency, the incremental stress-strain relationship was established and a modified Cam Clay model for structured soft clays was developed. Significant improvements were demonstrated in the performance of the new model by comparing with experimental data in the stress path tests for Bothkennar soft clay.