There is a slight black vertical line in the thumb nail is nail black line, may be nail fungal disease, nail trauma, nail mother mole and other diseases caused, need to be specifically analyzed. 1. Nail fungal disease: caused by a variety of fungi nail plate and (or) subnail tissue infection collectively referred to as nail fungal disease. Nail fungal disease is mainly caused by dermatophyte infection, followed by yeast and molds, which can cause false black lines on the thumbnail. 2. Nail trauma: if the nails are damaged by external force, the nails will appear black line, caused by the blood coagulation due to the trace bleeding within the nails, generally do not need to do special treatment, pay attention to the protection of nails in ordinary life. 3. Nail nevus: Nail nevus is caused by the proliferation of melanocytes in the nail area, which is clinically manifested by darker black lines on the nails, banded distribution, and wider range. There are many reasons why there is a slight black vertical line in the thumbnail, and it is recommended to go to the hospital for examination to clarify the cause of the disease and treat the symptoms in time.