Dermatofibroma is a common benign fibrohistiocytic tumor that usually appears as a firm, solitary nodule. Polypoid dermatofibroma is an uncommon variant which accounts less than 3% of all. Only a few cases of polypoid dermatofibroma have been reported. We report 37 year old man presented with a large mass on left knee which had been present for several years. The tumor size is 4x4 cm and slowly increased that it disturb patient daily activity such as changing a cloth. The tumor was resected for treatment and the histological examination confirmed to be a dermatofibroma. In our best knowledge, polypoid dermatofibroma in this case is the largest size in the literature.