The longer segmental dynamics of sub-Rouse modes in polystyrene with different molecular weights has been investigated by 2D correlation mechanical spectroscopy. The sub-Rouse modes were first separated from the alpha relaxation and Rouse modes, and their dynamics exhibits a similar change at the same temperature, T-B approximate to 1.2T(g), as the alpha relaxation. The relaxation time of sub-Rouse modes at T-B is independent of molecular weight and has a value of about 0.1 s, indicating that solely the time scale of the relaxation determines the change in dynamics of sub-Rouse modes. According to the coupling model, the change is caused by a strong increase in intermolecular cooperativity. The present work provides direct evidence for the intermolecular coupled nature of the sub-Rouse modes and demonstrates that the properties of the sub-Rouse modes resemble those of alpha relaxation, which could provide a new perspective for understanding the glass transition of polymers.