An implantable antenna is required with miniaturization and stable frequency performance when working inside human body. A novel implantable antenna is proposed, which has a step shaped symmetrical structure fed by Coplanar Waveguide(CPW). A cube shaped human body model filled with muscle phantom is utilized in simulation with the antenna in the center of body. The antenna still covers Industrial Scientific Medical (ISM) band. The simulation results show that the antenna still covers ISM band when it is planted in different human tissues. The proposed antenna is measured in the human muscle solution and the measured bandwidth is 2.02-2.5 GHz. The size of the proposed antenna is 4 mm×11.9 mm× 0.8 mm with FR-4 as substrate. Compared with the published implantable antenna, the proposed antenna has the advantages of small volume, wide bandwidth, and low production cost, etc. The proposed antenna can be applied in implantable medical equipments such as pacemaker, capsule endoscope, etc.