Since roaming was found as a new but common reaction path of isomerization, many of its properties, especially those of roaming transition state (TSR), have been studied on many systems. However, the mechanism of roaming is still not clear at an atomic level. In this work, we use first-principles calculations to illustrate the detailed structure of TSR in an inteal isomerization process of nitrobenzene. The calculations distinctively show its nature of antiferromagnetic coupling between two roaming fragments. Moreover, the effect of dispersion is also revealed as an important issue for the stability of the TSR. Our work provides a new insight into the TSR from the view of electronic structure and contributes to the basic understanding of the roaming systems.