In this Letter we report a neat method for preparing polymeric giant vesicles. Self-assembly of a rigid oligomer, polyether imide (PEI) with amino groups at its two ends and stearic acid (SA) in chloroform/cyclobexane, which was a selective solvent for SA, led to the giant vesicles. However, PEI and low-molecular-weight polystyrene with a carboxyl end (CPS) formed micelles instead of vesicles in the solvent mixture. Furthermore, morphology transition from micelle to vesicle was observed when SA was added to the micellar solution of PEI/CPS.