Abstract: A unified model for the distortional and lateral buckling of cold-formed Z-purlins subjected to bending is proposed in this paper. The model can study both the distortional buckling and the global buckling. The new model is different from Hancock's model. Two interactive springs between lateral and rotational restraints were introduced in this model. They were provided by the web and rest of the purlin to the flange-lip section. The Ritz method was used to acquire a closed-form solution. The elastic buckling coefficients of 20 purlin sections predicted by the solution showed an excellent agreement with those from Finite Element Method (FEM). Those results were also compared with the Hancock's method. The lateral bucking results given by the new model were compared with the Timoshenko critical moment as well, showing excellent agreement. The fitting formula for distortional buckling coefficients of Z-purlins was acquired, which was proved to have high-accuracy as compared to FEM results.