Phase equilibrium studies for semiconcentrated solutions of rodlike poly(gamma-benzyl L-glutamate) having oligo(ethylene glycol) as side chains (PBLG-g-OEG) have been investigated. The phase-boundary concentrations in isotropic and anisotropic phases for N,N-dimethylformamide (DMF) solution of PBLG-g-OEG with short side chains (PBLG2-g-380) are higher than those for solution of PBLG-g-OEG with long side chains (PBLG2-g-770), The lattice theory and the scaled particle theory for nematic solution, which don't distinguish the molecular architecture of the rodlike polymer, cannot explain this experimental result. Repulsive interaction between rodlike polymers by means of the attached side chains is proposed for the molecular orientation of PBLG-g-OEG in anisotropic state in order to describe the experimental result. Ternary phase diagrams of PBLG-g-OEG/poly(ethylene glycol) (PEG)/DMF show that the miscibility of rodlike PBLG-g-OEG and coiled PEG is most enhanced in the system of PBLG2-g-770, which has longest and largest amount of side chains. This experimental observation is explained by using the calculation based on the lattice theory and the repulsive interaction of side chains proposed above.