In order to investigate the relationship between limb bone internal structure and the aquatic adaptation, we examined humeri of multiple carnivoran species of various lifestyles (terrestrial, semiaquatic, and aquatic) using micro-focus CT scanner. The transverse section image passing through the inferred center of ossification is analyzed in order to estimate the structural remodeling in periosteal bone. Meanwhile, structural characteristics in sagittal section images of proximal end were described in order to assess the characteristics in endochondral bone. The degree of cancellousness around the center of ossification, which is interpreted as the amount of resorption in periosteal compact bone, is the lowest in the semiaquatic group and the highest in terrestrial groups. This suggests that the ratio of periosteal bone resorption in limb bones decreases in the first stage of the aquatic adaptation, possibly due to the less need for skeletal lightening in semiaquatic taxa than the terrestrial taxa, and then increases as the limb bones reduces their support function on land, presumably in relation to the deep diving. The bone trabecular architecture in humeral head is fine and well-oriented in terrestrial to semiaquatic carnivorans, while that of aquatic carnivorans is rather coarse and randomly oriented.