A small bulge on the arm may be caused by lipoma, sebaceous cyst, dermatofibroma and so on. 1. Lipoma: it is a common benign tumor of soft tissue, mostly occurring in the limbs and trunk of the subcutaneous, soft texture, can be moved, mostly painless. This can cause a small bulge on the arm. 2. Sebaceous cysts: commonly known as acne, refers to the sebaceous glands secrete too much sebum can not be discharged, the accumulation of the formation of cysts. Mostly in adolescent people, mainly manifested in the skin surface bulging, if complicated by infection, can cause redness, swelling, pain, fever and other symptoms. It can also cause a small bulge on the arm. 3. Dermatofibroma: a kind of intradermal benign tumor caused by fibroblasts or tissue proliferation. It is mostly related to mosquito bites and trauma. Often manifested as a single hard nodule on the skin, generally no conscious symptoms. It can also cause a small bulge on the arm. There are many other causes of a small bulge on the arm, should be taken seriously, timely consultation, identify the cause of the disease, and follow the doctor’s advice for further treatment or treatment.