When suitably o-substituted aromatic carbonyl compounds are irradiated, (n, π*) excited triplet state is produced. The carbonyl oxygen atom of the excited triplet state abstracts δ-hydrogen atom to give 1,5-biradical intermediates (Norrish type II reaction). The 1,5-biradicals cyclize intramolecularly to give benzofurans. This method is useful to prepare a variety of benzofuran derivatives. In this review, synthesis of benzofurans using photocyclization reactions of aromatic carbonyl compounds, solvent effect and substituent effect on cyclization step of 1,5-biradicals are described. The carbonyl compounds consist of 2-alkoxybenzaldehydes 4, 2`-alkoxyacetophenones 7, 2-alkoxybenzophenones 27, 2-formylphenoxyacetic acids 10, 2-acetylphenoxyacetic acids 11, ethyl 2-formylphenoxyacetates 14, ethyl 2-acetylphenoxyacetates 18, ethyl 2-benzoylphenoxyacetates 29, 8-alkoxy-l,2,3,4-tetrahydro-l-naphthalenones 32, 4-alkoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ones 36, ethyl 2-(8-oxo-5,6,7,8-tetrahydro-l-naphtyloxy)acetates 42,  ethyl 2-(5-oxo-6, 7, 8, 9-tetrahydro-5H-benzocyclohepten-4-yloxy)acetates 45, α-(2-acylphenoxy)toluenes 4e, 7e, 63e, 68e, 27e and ethyl 2-acylphenoxyacetates 14a, 18a, 80a, 85a, 29a. Photocyclization of a variety of aromatic carbonyl compounds gave benzofurans in moderate to good yields. Solvent and substituent effects played an important role on cyclization step of 1,5-biradicals. When less polar benzene was used cis-benzofuran was produced predominantly of two possible cis- and trans-isomers, showing high stereoselcctivity. Changing the solvent from benzene to polar acetonitrile and methanol, the stereoselectivity decreased dramatically because of hydrogen-bond between the hydroxyl group of 1,5-biradicals and solvent molecules. Photocyclization reactions of aromatic carbonyl compounds are useful method to synthesize benzofuran derivatives, especially, benzofurans which are difficulty to prepare by thermal reactions.