The iron-containing hemoglobins (Hbs) are essential proteins to serve as oxygen transporters in the blood. The cooperative oxygen-binding is a distinct feature for them to load and unload oxygen efficiently. Among various kinds of hemoglobins, the earthworm hemoglobins are the champions in carrying oxygen due to not only their large size but also the unusually high cooperativity. Here we report the first study on the cooperative mechanism in the Lumbricus terrestris hemoglobin. The near atomic resolution of our cryo-EM structure also enables us to further visualize the additional sinusoidal bracelet, which accounts for a new assembly mechanism of this enormous structure.