In one aspect of the disclosure, an intramedullary (IM) apparatus for treating a fractured bone of an animal includes an IM nail elongated along a longitudinal axis and having at least three locking holes equally distantly defined thereon along the longitudinal axis, and a positional reference member having a longitudinal vernier scale thereon. The IM nail is configured to be inserted to a medullary cavity of a bone fragment, such that the IM nail is fixable to the bone fragment by a locking member correspondingly inserted into one of the locking holes through the bone fragment. The positional reference member is configured to be detachably mounted to the IM nail by two positioning members inserted into two of the locking holes such that the longitudinal vernier scale is parallel to the longitudinal axis of the IM nail, and each of the locking holes is locatable by the longitudinal vernier scale.