Frequency and gate voltage dependences of capacitance in a C-60 field effect transistor (FET) showed an intriguing power law (C proportional to f(-p), p similar to 0.3-0.35) irrespective of the gate voltage. In order to interpret this phenomenon, we formulated a complex impedance of the bottom contact FET based on a distributed constant circuit model in cases of both a single grain channel and a multi-grain channel. The power law could be well explained in terms of the complex impedance formula using only a small number of fitting parameters, the results of which indicate the validity of the model. This kind of analysis could usefully characterize the organic FETs consisting of grain boundaries, providing information on the resistance ratio of the grain interior to the grain boundary. (c) 2006 Elsevier B.V. All rights reserved.