The relation between warpage and part thickness was investigated for a disk injection molded from PA66 compounded with 33 wt% glass fiber. A thin disk (1.5 mm thick) showed significant warpage, of magnitude 6.5 mm, while a thick disk (5.0 mm thick) did not warp. Finite element analyses were conducted, and the experimental results were numerically reproduced with 21% accuracy. It has been found that the thickness dependence of the disk warpage originates from the thickness dependence of the buckling temperature of the disk, and that quantitative disk warpage depends on the temperature of warpage measurement. Thickness change affects the amount of warpage but does not change essential warpage characteristics of the disk, characteristics such as inherent warpage deformation geometry and qualitative fiber orientation.