A black line on the nail may be caused by subnail hemorrhage, nevus of the nail, nail malnutrition and other reasons. 1. subnail hemorrhage: when the nails are extruded, impact and other traumatic injuries will lead to the rupture of capillaries in the nail bed, resulting in the occurrence of subnail bruising, appearing on the nails with a small black line of symptoms. Generally there is no need for special medication, with the growth of the nail bruises can slowly subside, but be careful to avoid re-squeezing, injury to the nail. 2. Nail nevus: small black lines on nails may also be caused by nail melanoma, which is mainly manifested as longitudinal pigmented bands with fuzzy edges, irregular and uneven pigmentation, and may be accompanied by symptoms such as ulceration, nail loss and nail destruction, which need timely surgical treatment to avoid deterioration of the condition. 3. Nail dystrophy: Nail dystrophy caused by the longitudinal ridge of the nail, can also be manifested as black lines on the surface of the nail, with the hand can be touched to the surface of the nail uneven, it is recommended to choose the oral multivitamin to carry out the treatment. There may be other reasons for the black lines on the nails, it is recommended that the patient to the hospital to clarify the cause, under the guidance of the physician for treatment.