Abstract：The Lode angle is often introduced into yield function or plastic potential function for accounting the complex mechanical behaviors, such as influences of intermediate principle stress and stress path on failure and deformation, of granular soil. Therefore, the derivative of Lode angle with respect to stress (denoted as ) must be involved for the calculation of plastic strain. Yet, the singularity of occurs at the stress states which correspond to the repeated principle stress, so it is difficulty to obtain the principle value of . A new method for calculation of principle value of are presented in the study，and some basic properties of are discussed in detail.
2012, Vol. 33 
