AbstractAn evaluation of degraded level is an important subject for industrial products. Ordinarily, many kinds of polymer material changes its color through the degrading process by altering its chemical structure, and IR and UV are applicable to ensure the production of the functional groups. However, these methods are hardly applicable to measurements of product on site without the sample collection. A spectrophotometer is sufficiently lightweight, compact, and capable of measurements on site without the sample collection. With these backgrounds, the correlation of the color with the tensile strength and the molecular weight was studied by using the spectrophotometer. Through the thermal aging test at various temperatures, specimens became yellowish and the increase rate of color change was different between neat Nylon‐6 and Nylon‐6 nanocomposite. Such changes were closely related to the tensile strength and the molecular weight. As a method to evaluate the polymer degradation, the correlation of the difference of yellowness index (ΔYI) with the tensile strength and the molecular weight was sufficiently high and the ΔYI indicated a difference between neat Nylon‐6 and Nylon‐6 nanocomposite. From these results, it is found that the color measurement with the spectrophotometer is a practical method for evaluation of polymer degradation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008