A black line on a molar tooth may firstly be a pigmentation, which can be treated by supragingival scaling. Secondly, it may also be caused by dental caries, which requires medication, surgery, and other treatments. 1. Hyperpigmentation: If hyperpigmentation is the cause of black lines on the teeth, supragingival scaling can be used to remove plaque, calculus and hyperpigmentation on the gums. Commonly used gingival scaling techniques include ultrasonic scaling and manual scaling. 2. Dental caries: For caries-induced black lines on the teeth, fluoride, silver ammonium fluoride and other topical medicines can be applied to the cavities for treatment. For patients with dental defects, it is also necessary to take composite resin direct bonding restoration, amalgam filling, inlay restoration. In addition to this, patients can also take remineralization therapy and other ways of treatment. It is recommended that the black lines on the teeth should be examined in time, and the doctor will take appropriate measures to treat the cause of the disease. Patients usually need to keep their mouths clean, need to develop the habit of brushing their teeth in the morning and evening, and need to rinse their mouths in time after each meal.