A banknote has been considered to be a prime medium of payment from past to present. The use of the banknote in daily life unavoidably cause more or less deterioration to its properties. An optical property of banknotes especially in the areas of security printing in of interest in this study. Hundred bath Thai banknotes are used became of their unique security features. A number of deterioration factors such as folding to several shapes, washing with different liquids and sacking with adhesive tapes are applied to the chosen banknotes. The optical method for investigations is the measurement of visible absorption using forensic light sources and spectrometer. The investigation reveals many interesting results such as which factor causing the most degradation to the security feature of banknotes.