It has been found that the warpage mode of an injection molded disk depends on the type of reinforcement of the material. To clarify the reason for the finding, correlations among the aspect ratio (lambda) of reinforcements, anisotropy in linear thermal expansion coefficient (alpha), and warpage have been studied for an injection-molded disk. The lambda is defined as the ratio of the length to the radius. The warpage of the disk is defined as the difference between the maximum and the minimum of the deformation at the free state, which is observed immediately after the disk is released from the mold. The study has been conducted with respect to the PA66 composites compounded with the reinforcement of lambda = 1 and lambda = 4. Ralpha = alpha11/alpha22 has been defined to represent the anisotropy of alpha, where alpha11 is the alpha in the flow direction, and alpha22 is that in the transverse to the flow direction. The flow is defined as the flow of the melted polymer in the injection molding. It has been experimentally found that Ralpha = 1.0 for unreinforced PA66, Ralpha < 1.0 for PA66 compounded with lambda = 4 reinforcement, and Ralpha < 1.0 for PA66 compounded with lambda = 1 reinforcement. The warpage mode in the Ralpha > 1.0 composite resembles a coffee cup, while the warpage mode in the Ralpha > 1.0 composite resembles a saddle. The results indicate that Ralpha determines the warpage mode of the disk. To prove that Ralpha is the adequate condition for determining the warpage mode, finite element analysis was conducted for the material of Ralpha < 1.0 and for the material of Ralpha > 1.0. It has also been found that the amount of warpage in the case of Ralpha not-equal 1.0 is approximately 10 fold that in the case of Ralpha = 1.0. On the other hand, the warpage of a disk molded from the blend of Ralpha < 1.0 composite and Ralpha > 1.0 composite decreases to the warpage level of the disk of Ralpha = 1.0. It is deduced that Ralpha is the important parameter for controlling the warpage since Ralpha < 1.0 and Ralpha > 1.0 represent the two counteracting material properties in warpage.