Uneven red blood cell size means an increase in the width of the average volume distribution of red blood cells, and the most common causes are iron deficiency anemia and megaloblastic anemia. Immature erythrocytes have a nucleus and a cell plasma, which is also known as hemoglobin and consists of heme and pearl protein, which is composed of iron and protoporphyrin. When the synthesis of cytoplasm decreases, it causes the head of red blood cells to become smaller and the central light-stained area to expand, resulting in microcytic hypochromic anemia, which is commonly caused by iron deficiency, excessive iron loss, and abnormal synthesis of pearl protein. The main raw materials of the nucleus are folic acid and vitamin B12. When the raw materials are lacking, the nucleus cannot be synthesized and the development of red blood cells will be hindered, and the red blood cells will stay larger in size, producing larger red blood cells, which is called megaloblastic anemia. Therefore, uneven red blood cell size indicates some abnormalities in the patient’s body, and early medical attention is recommended.