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The degradation process in pentacene-based organic thin-film transistors (OTFTs) is investigated. Pentacene-based OTFTs were fabricated with and without octadecyl trichlorosilane (OTS) treatment, and their device characteristics during lifetime test are evaluated using low-frequency-noise (LFN) spectroscopy. It is found that the devices exhibited the 1/f type of noise behavior with generation and recombination noise superimposed. The drain-current noise was found to vary proportionally with drain current according to Hooge's empirical relation of flicker noise. Devices without any treatment show obvious interface traps and deep-level traps, while devices with OTS treatment show nonexistence of interface traps and suppression of deep-level traps. The LFN intensity is found to decrease during the device lifetime test initially, while upon the device failure, the noise level is observed to increase again. The viability of using LFN as a diagnostic tool in the organic transistor is demonstrated.