A black vertical line on the thumb nail may be caused by subnail hematoma, onychomatous nevus, onychomelanoma, etc. Subnail hematoma and onychomatous nevus are generally not serious. Nail melanoma is a malignant tumor, which is more serious and requires timely consultation. 1. Subungual hematoma: often caused by trauma, hematoma can be purple or black, black old hematoma can be manifested as black vertical lines on the thumbnail, which usually can be cured by itself and is generally not serious. 2. Nevus of mother of nail: nevus of mother of nail is a melanocytic nevus at the mother of nail, the metabolism of mother of nail pushes forward the growth of nail plate, thus forming a black-brown strip under the nail plate, the boundary is regular, clear, and the color is uniform, and the nevus of mother of nail is stable and no longer develops in the adulthood. Nail nevus is benign tumor, usually not serious. 3. Nail melanoma: middle-aged people’s thumb is easy to be involved, patients often have history of trauma, clinical manifestations: longitudinal black nail, its pigment band is often dark brown or black, the edge of the band is fuzzy, the distal end of the band is narrower than the proximal section, suggesting superficial dissemination of the tumor, this disease is a tumor, it is more serious, and need to be timely consulted. There may be other reasons for a black vertical line on the thumbnail, so it is recommended that you go to a regular hospital, clarify the cause of the disease, and follow the doctor’s instructions for standardized treatment.