By means of light scattering, unimolecular-micelle formation is investigated for poly(methyl methacrylate)graft-poly(styrene)s with different graft-crain densities in a dilute solution of a selective solvent. Molecular weights of the poly(methyl methacrylate) (PR?MA) backbone and the branch poly(styrene) (PS) are about 6 x 10(6) and 9 x 10(3), respectively, and the graft-chain density is relatively low, ranging from 6 to 17 of PS composition in wt%. The solvent used is iso-amyl acetate, which is a thermodynamically good solvent for PS and a poor solvent (theta-temperature : 60 degrees C) for PMMA. Radius of gyration, hydrodynamic radius, and the second virial coefficient are measured as a function of temperature ranging from 10 to 70 degrees C. At a branch PS composition of 6 wt%, with decreasing temperature, the copolymer chain shrinks in the same manner as the PMMA chain with no PS branches, although the theta-temperature decreases by about 25 degrees C. This indicates that no particular order structure is formed. At higher branch densities, rod-like unimicelles are strongly suggested to be formed by intramolecular segregation between the PMMA backbone and PS branches, with the shrunken PMMA backbone forming the core rod covered with PS chains. With decreasing temperature, the dimension of the unimolecular micelle decreases probably due to decreasing contour length of the micelle core.