The assemblies and transformations of dendritic polyethylene (DPE)-poly(oligo(ethyleneglycol) methacrylate) (POEGMA) amphiphilic micelles have been demonstrated by cryo-TEM and DLS techniques under elongation flow stimuli. The flow rate-dependence of the dissymmetry ratio suggests the possibility that a combination of shear and elongation could also be responsible for the transitions of DPE-POEGMAs, but it is obvious that the exposure of elongation flow is essential and plays a key role in the assembly and fusion of the DPE-POEGMA micelles. Fluorescence resonance energy transfer (FRET) is used to provide insight into the assembly and fusion of DPE-POEGMA under elongation flow. The FRET results show that a shorter separation distance of DiO-DiI with higher elongation rate can result in higher FRET efficiency. Furthermore, DPE-POEGMAs can display the responsive switching ability of the elongation flow-triggered FRET.