A series of polymacromonomer samples (F15) consisting only of polystyrene and having a fixed side chain length of 15 styrene residues have been prepared and studied by static light scattering in cyclohexane at different temperatures and in toluene at 15 degrees C. The measurement has also been made on polymacromonomer samples (F33) of the same kind but with 33 side chain residues in toluene. The second virial coefficient for the F15 polymer in cyclohexane vanishes at 34.5 degrees C, the Theta temperature for linear polystyrene, as was previously found to be the case for the F33 polymer. Analysis of the measured mean-square radii of gyration based on the wormlike chain with or without excluded volume shows that, while the contour length per main-chain residue is insensitive to the side chain length, the Kuhn segment length lambda(-1) (or more generally the stiffness parameter in the helical wormlike chain) under the Theta condition remarkably increases with increasing side chain length and that the lambda(-1) values (16 and 36 nm) for the polymacromonomers F15 and F33 in toluene, a good solvent, are about 1.6 times as large as those in the Theta solvent. Thus, it is concluded that, in addition to the high segment density around the main chain, repulsions between the main chain and side chain and between neighboring side chains play an important role in the high stiffness of polymacromonomers.