An improved method for determination of extrinsic resistances for 70 nm InP high electron mobility transistor （HEMT） is proposed in this paper. A set of expressions have been derived from the equivalent circuit model under operating bias points （V_gs > V_th， V_ds = 0 V）. The extrinsic resistances are iterative determined using the discrepancy between simulated and measured S-parameters as an optimization criterion using the semi-analytical method. Good agreement between simulated and measured S-parameters under multi bias over the frequency range up to 110 GHz verifies the effectiveness of this extraction method.