There is a round lump inside the outer side of the arm may be related to dermatofibroma, lipoma, epidermal cyst and other diseases. 1. Dermatofibroma: the etiology is not clear, may be related to trauma, insect bites and other factors. It is mainly manifested as a round firm nodule, mostly seen on the outer side of the limbs. 2. Lipoma: the etiology is not clear, may be related to heredity, metabolic abnormalities and other factors. One or more lumps will appear under the skin of the patient, with soft texture, mostly seen in the limbs, trunk and other parts of the body. 3. Epidermoid cysts: the etiology may be related to trauma, embryonic ectopic and other factors. Patients will appear round or ovoid nodules inside or under the skin, which can be seen in any part of the body. If there is a round lump inside the outside of the arm, it is recommended to go to the regular hospital for examination and to clarify the cause of the disease.