The asymmetry between lower and upper intermodulation distortions (IMD) in a heterojunction bipolar transistor (HBT), which can be observed under certain baseband termination and bias conditions, is analyzed using a large-signal model. This approach enables harmonic balance analysis of IMD asymmetry in nonlinear circuits using HBTs and, therefore, overcomes the limitations of the conventional Volterra-series analysis. Closed-form expression derived from Volterra analysis is also employed to present comparisons with the large-signal model approach. Measurements indicate that the asymmetry is very sensitive to baseband termination, which varies with the envelope frequency of the input RF signal. Specifically, zero crossings of asymmetry are observed at each resonance frequency where the reactive part of baseband termination changes its sign rapidly. It is also found that the asymmetry is enhanced at a specific small bias current where the dominance of the 3rd order nonlinearity is greatly reduced by cancellation with the 2nd order nonlinearity. The bias condition for asymmetry occurrence is calculated, using a simple analytical equation, to be related to measurement data. All the measurements are verified successfully by nonlinear simulation using large-signal model. Finally, the advantage of the large-signal model approach compared to Volterra analysis for asymmetry prediction is discussed.