In this paper, we analyzed the fine structure of the rotational discontinuity (RD) in the magnetopause of the open magnetosphere by using the MMS four-point magnetic field measurements. It is found that RD is very common within the magnetopause when reconnection occurs at the magnetopause. Furthermore, RD usually occurs closer to the magnetosheath side of the magnetopause. RD is very thin and its thickness is usually much smaller than 0.1 Re. The magnetic field rotation maximums and magnetic strength minimums within the RD. The radius of curvature of the magnetic field lines reaches its minimum value in the RD (~0.03-0.62 Re). In addition, the radius of curvature of magnetic field line of RD is usually larger than the thickness of RD structure, indicating that the magnetic field lines are partly lying in the RD. Generally，the field-aligned current dominates in the RD.