Petechiae and purpura are both hemorrhagic diseases, the difference between them are: the size, color and bleeding site of petechiae and purpura are different, petechiae is called petechiae when the bleeding range is more than 5 mm, and the color of petechiae is usually blue-purple, the common site is limbs and trunk area. In contrast, purpura is 3-5 mm in size, the color is usually bright red at first, then gradually becomes lighter and finally becomes yellowish brown, the bleeding area is mostly seen in the extremities, sometimes the mucous membrane can also appear purpura. At the same time, there is a difference between the diseases that cause petechiae and purpura: the most common diseases that cause petechiae are coagulation disorders, such as hemophilia, vascular hemophilia and so on. The most common diseases that cause purpura are allergic purpura, immune thrombocytopenic purpura and so on.