1,4-dihydropyridine　undergoes　a　head-to-tail　[2+2]　photodimerization　that　leads　to　the　formation　of　anti-dimer,　syn-dimer　and　cage　dimer　in　solution.　Their　crystal　X-ray　crystallographic　analysis　suggests　that　the　stereoisomers　in　solution　are　restrainted　by　the　relative　position　of　double　bonds　of　intramolecules,　hydrogen　bonds　and　steric　hindrances,　which　are　different　in　the　solid　state.　The　cage　dimer　is　formed　by　the　intramolecular　reaction　of　syn-dimer,　and　is　topochemical　controlled　by　the　nearest　distance　between　potentially　reacting　double　bonds,　rotational　angles　of　the　building　cyclobutane　ring　and　the　orientation　of　carbonyl　ester.