Pentacene thin film transistors (TFTs) were made using different deposition tools and patterning methods to confirm the degradation of the octadecyltrichlorosilane (OTS) monolayer due to plasma damage during sputter deposition or to thermal damage during the evaporation deposition process. The degradation of the OTS monolayer was confirmed by studying the morphology of pentacene grown on the OTS monolayer and the electrical properties of pentacene TFTs. The OTS monolayer on SiO2/N++Si substrate was not degraded by thermal energy during evaporation, but it was affected by plasma damage during sputtering. Pentacene grown on the OTS monolayer did not show well-defined grains when the OTS monolayer was exposed to plasma, and the performance improvement in the pentacene TFT was not observed. In the lift-off method, the OTS monolayer was covered with photoresist, which could prevent sputtering damage, and OTS treatment improved the performance of the pentacene TFT.