A decrease in the average volume of red blood cells is usually associated with microcytic anemia. Some of the more common microcytic anemias are iron deficiency anemia, thalassemia, and iron-granulocytic anemia. The most common is iron deficiency anemia. Iron deficiency anemia is mainly due to the decrease of iron in the body and the inability to synthesize hemoglobin, so the hemoglobin in each red blood cell decreases and the corresponding volume of red blood cells decreases. If it is clear that it is iron deficiency anemia, it is recommended that the patient take oral iron supplementation therapy and also the diet should be supplemented with iron to help the red blood cells synthesize hemoglobin normally. The number of red blood cells is directly proportional to the number of hemoglobin. After the number of hemoglobin rises, the volume of red blood cells will gradually be normalized. Thalassemia, on the other hand, is caused by a disorder in the synthesis of the peptide chain of pearl proteins, which does not allow the production of normal hemoglobin, resulting in a low red blood cell volume. This is a genetic disease and there is no specific treatment for it. Mild cases can be monitored, while severe cases require component blood transfusions, stem cell transplants, etc.