The contact-size-dependent characteristic of cutoff frequency fT in bottom-contact organic thin film transistors (OTFTs) is studied.The effects of electrode thickness,field-effect mobility,channel length and gate-source voltage on the contact length (source and drain electrodes’ length) related contact resistance of bottom-contact OTFTs are performed with a modified transmission line model It is found that the contact resistance increases dramatically when the contact length is scaled down to 200 nm.With the help of the contact length related contact resistance,contact-size-dependent f T of bottom-contact OTFTs is studied and it is found that f T increases with the decrease of the contact length in bottom-contact OTFTs.