Complexation between C-60-end-capped linear or four-arm poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) was studied. The introduction of hydrophobic [60]fullerene (C-60) in PEO has a dramatic effect on the complex formation of the PEO/PAA system. Interestingly, the yields of C-60-end-capped four-arm PEO/PAA complexes are lower than those of C6a-end-capped linear PEO/PAA complexes. The result indicates a competition between the hydrophobic effect and the steric effect originated from C-60. Similar to the PEO/PAA system, there are hydrogen-bonding interactions between the carboxylic acid groups of PAA and the ether oxygen in C-60-end-capped PEOs.