Abstract: In this paper, a failure criterion for unsaturated loess is set up according to the deposit history and strength characteristics of loess. It satisfies the Mohr-Coulomb failure criterion on p-q plane and the whole condition of shape functions on π plane. All parameters can be determined for this failure criterion with conventional triaxial compression tests and triaxial extension tests. The method for determining the effective stress parameters is presented according to the new effective stress expression of unsaturated soil under three-dimensional stress state. Moreover, the adaptability of the failure criterion for loess is verified with experimental data from true triaxial tests on intact unsaturated loess. It is found that the failure criterion is better than other failure criteria, such as the improvement criterion, double circular slip criterion, and Willam & Warnke criterion.