Differently shaped carbon fibers (R-, I-, C-, Y-, and X-type) were prepared from melt-spinning of reformed naphtha cracking bottom oil precursors through various shaped spinnerets. These carbon fibers were activated by steam and activation properties were compared. The decrease of hydraulic radius resulted in the extending of the external surface area of carbon fibers. Activation energy and rate of differently shaped carbon fibers were affected by external surface area. Especially, the activation rate of tetralobal carbon fibers (X-type) appeared much larger than other shaped carbon fibers due to the smallest hydraulic radius. Adsorption capacity of tetralobal activated carbon fibers was also larger than other shaped activated carbon fibers.