An approach for determination of small-signal equivalent circuit model elements for InP HBT is presented in this paper. The skin effect of the feedlines is taken into account in the proposed model. This method combines the analytical approach and empirical optimization procedure. The intrinsic elements determined by a conventional analytical parameter transformation technique are described as function of extrinsic resistances. An excellent fit between measured and simulated S-parameters in the frequency range of 2~110 GHz is obtained for InP HBT.