Photovoltaic (PV) backsheets composed of polyvinylidene difluoride (PVDF) and polyethylene terephthalate (PET) films were evaluated via principal component analysis (PCA) of near infrared (NIR) spectral datasets obtained using a potable NIR spectrometer. The discrimination of material and the degree of deterioration of the backsheets following heating and humidifying or irradiating with UV light was examined. Multiple linear regression (MLR) analysis was also employed to evaluate the degree of deterioration. The reference values (peel and tensile strength) were obtained via the International Electrotechnical Commission (IEC) 61215 and Underwriters Laboratories (UL) 1703 standard methods. The values of the peel strength and tensile strength after heating/humidifying treatment correlate quite strongly with the NIR predicted values (R2=0.946 and R2=0.973, respectively). These results show that the degree of deterioration of a PV backsheet can be nondestructively and easily determined via NIR spectroscopy. A Deterioration of backsheet for photovoltaic (PV) devices is recently evaluated by its mechanical strength. A quick and nondestructive evaluation method for the deterioration of the backsheets is required, since the present method is destructive. The evaluation of the deterioration of the PV backsheet by means of near-infrared(NIR) spectroscopy have been studied. It was found that the principal component analysis (PCA) of NIR spectra is very effective for the discrimination of the degree of the deterioration of backsheets. The mechanical strengths predicted by multiple linear regression (MLR) of NIR spectra were strongly correlated to the reference values by the standard method. These results show that the degree of deterioration of a PV backsheet can be nondestructively and easily determined via NIR spectroscopy. Display omitted ► The deterioration of the photovoltaic backsheets was evaluated by NIR spectroscopy. ► Principal component analysis allowed the discrimination of material of backsheets. ► The mechanical strengths predicted by NIR were strongly correlated to the reference values.