Time-resolved laser light scattering, combined with transmission electron microscopy, was employed to study the kinetics of the sphere-to-rod transition. The brush-rod block copolymer based on poly[poly(ethylene glycol) monomethyl ether methacrylate] and poly{(+)-2,5-bis[4'-((S)-2-methylbutoxy)phenyl]styrene}, PEGMA(37)-b-MBPS141, formed spherical large compound micelles (LCM) in mixed solvent of THF and water. LCM underwent the transition to large compound rod (LCR) in the time scale of hours, and the transition was favored at higher polymer concentration and at intermediate water content. It was also found that a large amount of free polymer chains existed throughout the transition process. The depletion force generated by the free polymer chains was estimated to be in the order of 0.1kT. Therefore, besides the instability or defects of the LCM, the depletion effect also made a positive contribution to the sphere-to-rod transition.