A rare case of primary intraosseous squamous cell carcinoma(PISCC) arising from lining epithelium of a dentigerous cyst is described. The case occurred at the left mandibular 3rd molar region in a 56-year-old Japanese woman. Clinical observation revealed cyst formation with an impacted 3rd molar, a common finding in dentigerous cyst, in the left mandible. Histopathologically, the lining epithelium of the cyst demonstrated transition from epithelial dysplasia to invasive squamous cell carcinoma(SCC). This case was diagnosed as PISCC arising from lining epithelium of a dentigerous cyst.