Supramolecular polymerization of a naphthalene-azoben-zene dyad bearing a barbituric acid hydrogen-bonding unit was studied in nonpolar media. The length of supramolecular polymers could be regulated by changing the assembly conditions in terms of kinetics. Supramolecular polymerization initiated by cis-to-trans photoisomerization afforded shorter supramolecular polymers with narrower polydispersity compared to those generated by thermal polymerization.