A novel flexible implantable antenna is proposed. The antenna is a half cutting of a Coplanar Waveguide(CPW)-fed antenna with symmetric geometry. The half-cut antenna is optimized and analyzed in a single-layered body phantom by using HFSS. Simulation shows that there are frequency bandwidth shifts when the body phantom is filled with different tissues of muscle, small intestine, and stomach, respectively. The antenna bandwidth covers Industrial, Scientific, and Medical(ISM) band in the case of skin, muscle and small intestine. Simulation also shows that the antenna bandwidth has little change with the varying of substrate thickness. Measurements are done by arranging the antenna in meat mince and skin-mimicking gel. For the two cases, the measured |S11|<-10 dB bandwidth of the proposed antenna is 1.91-2.65 GHz and 2.19-2.79 GHz covering ISM band, respectively. The proposed antenna has a small size of 6 mm×13.5 mm×24 μm. The proposed antenna is also investigated with a conformal structure adhering to a cylinder. The measured bandwidth of the conformal antenna is 2.26-3.11 GHz. Compared with published literatures, the antenna has advantages of low profile, light weight and wide bandwidth. The proposed antenna can be applied in wireless implantable medical devices, such as capsule endoscope, wireless cardiac pacemaker etc.