Buckling is the effect of part instability due to compression forces. When external compressive forces are applied – sudden deflection can occur without destruction of the part. In packing stage of injection molding process significant pressure is applied to melted polymer. It is used to compensate polymer shrinkage that occurs during solidification of polymer and is used to reduce the warpage of the part (which comes from differential shrinkage). That compressing pressure is frozen within part when polymer becomes solid and is present in the part after removal of the part from injection mold. Compressive stresses that was introduced during packing stage can lead to the buckling of a part, even if there are no external compressing forces applied to the part, especially when the thickness of the part (and its stiffness) is small. In this paper buckling of thin-walled injection molded parts was investigated. The research was conducted with numerical simulations performed with Autodesk Moldflow Insight software to obtain knowledge about the relation between geometry of the part and its stability.