Dentigerous cyst is a cyst arising by tissue fluid accumulation between the enamel and the residual enamel epithelium. It includes the crown of the impacted tooth and excludes the root. The treatments of the dentigerous cyst are enucleation, marsupialization, decompression, surgical excision and etc. In these cases children were evaluated for the chief complaints of the remained mandibular primary second molar. Each cases showed swelling on the buccal side of the primary secondary premolar. In the radiologic evaluation a radiolucent lesion including the crown of the mandibular secondary premolar was found, and it was diagnosed as dentigerous cyst. And the histopathologic examination showed the same result. Marsupialization was operated through alveolar socket and the alveolar socket was protected with vaseline gauze, the obturator combined with space maintainer was followed by the operation. After the operation, the impacted permanent tooth was showing faster eruption speed than the other normal teeth.