In this Article, we report the first study of the molecular dynamics in the glass rubber transition region in polyisobutylene by 2D correlation dynamic mechanical spectroscopy (2DC-DMS). With the help of the high resolution and high sensitivity of the technique, the sub-Rouse modes are independently separated from the Rouse modes and local segmental motion (LSM). According to the positions and widths of autopeaks of three modes of molecular motions, the loss tangent peak is resolved into three peaks by nonlinear fitting method. Moreover, the glass rubber transition region is divided into three regions. 2DC-DMS has been demonstrated to be a powerful tool for studying the molecular motions with different time/length scales.