Contrary to conventional Si-based device, organic thin-film transistors (OTFTs) constituting organic semiconductors are easily destroyed. Investigating failures using traditional FA techniques like electron microscopy or cross section is hard to procure. Therefore, the purpose of this paper attempts to develop approaches for failure analysis of OTFTs. We successfully demonstrate a non-destructive technique for defect inspection and localization, exactly specifying the failure of OTFT with a void existing in the organic dielectric. Application of optical beam induced resistance change (OBIRCH) in current leakage localization is shown. Non-distorted cross-section sample preparation and analysis of internal structure by electron microscopy are also developed. Moreover, the failure of OTFT induced by high driving voltage punch-through is analyzed using techniques established in the article. Particular elongation of the gate metal was found to be the failure mode.