This reports on the direct thermal observation of the pentacene-based Organic Thin-Film Transistors (OTFT) under the real operating conditions. An electrical technique which employs a thermal transient method was utilized for the investigation of determination of inner structure of device. Applying different input power, the temperature variation in the channel was determined. Under the driving power of 0.226 W, a thermal resistance of 240 degrees C/W was measured for the OTFT package. It was demonstrated that the heat generation inside the OTFT package increases with the input power to the OTFT package. Thermal distribution inside the OTFT package was mapped using a CFD (Computational Fluid Dynamics) method for the various input power. The results indicate that the design with effective thermal dissipation is imperative for reliable operation of the OTFT package. (C) 2010 Published by Elsevier B. V.