A pigmented nevus with a diameter of 3mm may be a melanoma, and further examination is needed to confirm the diagnosis. Benign naevus is usually <3mm in diameter, with neat edges, smooth surface, and relatively constant size and color. Malignant melanoma usually grows rapidly in a short period of time, with a diameter of ≥3mm, localized elevation, changes in shape and color, and even localized hemorrhage. If a nevus with a diameter of 3mm has the above characteristics, it is necessary to rule out melanoma. However, it is not accurate to judge by symptoms alone, and if a nevus is suspected to be malignant, histopathological examination of the skin lesions and immunohistochemistry are needed to make a clear diagnosis and exclude the diagnosis. For melanoma, since it is a malignant disease of the skin, once diagnosed, it should be treated by surgery, radiotherapy, chemotherapy, molecular targeting drugs and other active treatments according to doctor's instructions, and at the same time, it should also pay attention to sun protection, wear a hat or umbrella when going out, and it can be self-checked on the skin in order to facilitate the early detection of lesions. If you suspect that the nevus is malignant, it is recommended that you go to the hospital in time, after clarifying the cause of the disease, under the guidance of the doctor for symptomatic treatment.