Abstract: Hygrothermal influences on the mechanical properties of the fiber-reinforced, symmetrical cross-ply composite laminates with a through-width delamination are analyzed from the point view of micro-mechanics of composite materials. Governing equations for the buckling of the delaminated laminate subjected to the in-plane loading under the hygrothermal environments are educed using the macro-micro analytical approach. Using separation of variables in space, the governing equations are solved and the analytical results are obtained by satisfying the continuity and equilibrium conditions at the delamination fronts. Numerical calculations are performed to comprehensively discuss the influences of the geometric parameters, the material properties, the temperature, and the degree of moisture concentration on the buckling performances of the symmetric cross-plied laminates.