An efficient procedure for the synthesis of 6,7-dichloro-5,8-phthalazinedione (4) was developed in 49% overall yield via chloroxidation of 5,8-diaminophthalazine (8). And a series of its derivatives, 7-pyridinium-5,8-phthalazinedione-6-oxide (9), 6-chloro-7-phenylamino-5,8-phthalazinedione (10), 6,6-dimethoxy-6H-2,3,6b,11-tetraazabenzo[a]fluoren-5-one (11a), and 6,6-diethoxy-6H-2,3,6b,11-tetraazabenzo[a]fluoren-5-one (11b) have been synthesized.