A novel series of poly(p-phenylene)s (PPPs) with polyhedral oligomeric silsesquioxanes (POSSs) on their side chains was prepared. The obtained POSS-modified PPPs are as follows: 25POSS-PPP (2b, containing 25 mol-% of POSS units in all side chains), 50POSS-PPP (2c, containing 50 mol-% of POSS units in all side chains), 100POSS-PPP (2d, containing 100 mol-% of POSS units in all side chains), and 0POSS-PPP (2a, as a blank polymer). Films polymer 2d showed the same absorption and photoluminescence (PL) spectra as those in CHCl3 solution, indicating that bulky POSS units strongly suppressed intermolecular aggregation of the PPP backbone. Polymer 2d showed the same PL spectra even after thermal annealing at 150 degrees C for 6 h. This enhancement of PL stability is due to the significant effect of the bulky POSS units.