We present a simple analytical model for the drain current dependent cutoff frequency fT in submicron high frequency MOSFET's. The model combines the SPICE MOS3 drain current model and a modified McMacken-Chamberlain model for the current dependent transit time, which allows the description of the I-V characteristics and fT vs. current with the same set of SPICE parameters. The model is verified by comparing simulated results to measured data for 0.6 μm RF MOSFET's fabricated at VTT Electronics. The model explains well the observed behaviour. Furthermore, the comparison indicates that when the channel length is reduced to a half-micron range and below, most of the charging delay is caused by the transit time.