Radicular cysts are the most common inflammatory cysts and arise from the epithelial residues in the periodontal ligament as a result of periapical periodontitis following death and necrosis of the pulp. They are usually encountered in association with permanent teeth. However, occurrence in relation to deciduous teeth seems to be very rare. In this case, we presents treatment of radicular cyst associated with mandibular primary second molar following pulp therapy and discusses the relationship between pulp therapy and the growth of the cyst. The treatment consisted of decompression of the cyst sac, extraction of the involved primary tooth and 6 months follow up of the patient. Our purpose of the study was to evaluate the bone formation after the decompression and describe the benefit of this conservative surgical treatment. Case Report: A 7 years old girl, radiographic examination showed a large, circular, well-defined unilocular radiolucent area on the left mandible and impacted #34 and #35 which had incomplete roots and open apices. Signs of root resorption were evident in the adjacent teeth. Under local anesthesia, mandibular primary left first (#74) and second molars (#75) were extracted and decompression of the cyst was performed. A tissue sample was also taken for the biopsy. Conclusion: The follow-up was performed periodically under radiographic examinations and after 6 months the impacted teeth were at the position and new bone formation was evident at the former cyst site. This surgical technique preferred in young patients to promote the bone formation and encourage the eruption of permanent teeth.