The blackening of teeth is caused by the following reasons: 1. The more common reason is the occurrence of tooth decay. The tooth decay usually occurs in the sulcus point gap or the adjacent gap of the front teeth. Patients can see the darkness inside the teeth by looking at the mirror, and need to solve this problem by filling treatment. 2. There are dental stains attached to the surface of the teeth. If the patient has the habit of smoking or likes to eat colored things, such as chocolate or like to drink cola, coffee, etc. may have stains attached to the surface of the teeth, need to clean teeth to remove. 3, the patient ate tetracycline drugs in childhood, may lead to teeth dentin black and yellow, is no way to solve, can consider the day after bleaching, etc., as far as possible to whiten teeth.