In this paper, we simulate the nonlinearity of a multifinger heterojunction bipolar transistor (HBT) operated at radio frequency (RF). We directly solve the nonlinear differential equations of the HBT large-signal model with the electrical-thermal feedback equations in time domain using the waveform relaxation (WR) and monotone iterative (MI) methods. The temperature dependence of energy band gap (Eg), current gain, saturation current and thermal conductivity are also taken into consideration. With the developed simulator, the power-added efficiency (PAE), 1-dB compression point (P1-dB) and output third-order intercept point (OIP3) of a three-finger HBT are calculated. Our results illustrate the effects of self-heating and thermal coupling among different fingers play important roles in the nonlinearity of the multifinger power transistors. Furthermore, the proposed method allows us to evaluate the thermal effects on linearity of the multifinger power transistors and perform optimum design for these devices.