An extendable bat has a covering member, a tubular core, a handle, an adjusting device, a cap, and a collar. The adjusting device has a first locking member and a second locking member. With the cooperation of the first locking member and the second locking member, the length of the bat can be adjusted and the handle is held in place at a desired length. Accordingly, the cost for manufacturing the extendable bat is lowered.