Organic molecular semiconductors, such as pentacene (C22H14), is of great interest for potential application in thin-film organic field-effect transistors (OFETs). Novel scanning Raman microscopy was applied to the analysis of OFETs. Effective channel sizes in top-contact and bottom-contact OFETs were determined from scanning Raman spectra. The bottom-contact OFET showed a sharp change in Raman intensity at the gold/pentacene interface over a short distance while the top-contact OFET showed a gradual change in Raman intensity over a wide region. In the case of a gold electrode deposited on pentacene, the real channel size could be as small as half of the intended size. This resulted in the decrease in effective channel size and thus in the overestimation of the calculated mobility. The small channel length could be attributed to the presence of gold (composite) inside the channel in the top-contact OFET.