Lump on the small arm pressure pain without trauma consider with sebaceous cysts, lipoma, nerve sheath tumor and other causes. 1. Sebaceous cyst: It is a mass formed when the opening of sebaceous gland is blocked. There is a thick membrane on the outside, which contains a tofu scum-like substance inside. Sebaceous cysts are susceptible to secondary bacterial infections, which can lead to the formation of abscesses. Therefore, once diagnosed, early surgical removal is recommended. 2. Lipoma: Abnormal proliferation of subcutaneous fat leads to the formation of lumps. If the lipoma is accompanied by pain, surgical excision is feasible, and after excision, it is routinely sent to the Department of Pathology for examination. 3. Nerve sheath tumor: abnormal proliferation of fibroblasts in the nerve sheath under the skin of the arm leads to the lesion of local nerve sheath tumor. It causes hard lumps to appear under the skin of the arm, and pain symptoms arise when pressed. When there are hard lumps under the skin of the arm, and pain when pressed, it is recommended to go to the hospital in time to find out the cause of the disease after symptomatic treatment.