Modular multilevel converter (MMC) has complex internal dynamics, such as capacitor voltage fluctuations and harmonic circulating currents, which may have a big impact on the terminal behaviors of the MMC. Therefore, it is significant to include these internal dynamics in the MMC modeling. To do this, the harmonic state space (HSS) based modeling method is applied in this work. The proposed MMC models are validated by time-domain simulations. Furthermore, the dynamic harmonic response and stability analysis of the MMC are also carried out using the harmonic transfer function (HTF).