In this paper, an implantable antenna for the 915 MHz and 2.45 GHz Industrial, Scientific and Medical (ISM) bands is proposed. It is composed of a cut-edge spiral connected with a flag-shaped structure which are fed by a 50  microstrip line. The ground is also composed of a flag-shaped part connected to a thin strip. The antenna gain was enhanced in two stages by incorporating a flag-shaped structure, first into the patch and then into the ground. Maximum 3D gains of -20.1 and -30.7 dBi have been obtained at 915 MHz and 2.45 GHz, respectively which is large enough to build up a communication link over 10 m considering a link margin of larger than 5 dB. The antenna's compact size, favorable radiation characteristics, and distinctive structure for gain improvement makes it well- suited for implantable applications.