There are many causes of small black grains on the penis. Common causes include melanocytic nevi, genital herpes, etc. They can be treated with medications such as antiviral drugs as well as surgery. 1. Melanocytic nevus: It is a benign melanocytic tumor that can appear on the skin and mucous membranes of any part of the body. The lesions are of different sizes, with clear boundaries, and the damage starts as a patchy rash and then can form papules and so on. 2. Genital herpes: Genital herpes is a sexually transmitted disease caused by herpes simplex virus infection, which is easily repeated and difficult to cure. The main clinical symptoms are blisters and ulcers in the genital area, which can be accompanied by itching and pain. In addition, there are many other causes that can cause similar symptoms. Therefore, when discomfort occurs, you should go to the hospital immediately.