A simple, efficient, and convenient method for the photodimerization of 4-aryl-1,4-dihydropyridine in room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) has been developed. By this way, head-to-tail syn-dimers and 3,9-diazatetraasteranes were obtained in good to excellent yields. In addition, [Bmim][BF4] can be reused at least 5 times as the medium without obvious decrease in reaction time and yield.