A magnetodielectric substrate is firstly proposed for implantable antennas in medical implant communication service (MICS) band. Ba₃Co₂Fe₂₄O₄₁ (Co₂Z), Z-type hexaferrite flakes synthesized by molten-salt method were incorporated with a polymer matrix, having a real permeability of 2.88 and a magnetic loss of 0.029 at 402 MHz. A spiral antenna with the magnetodielectric substrate has a bandwidth 70% and 50% broader than that with a commercial dielectric substrate, Rogers 3210, in a 2/3-phantom model and a multi-layered body model, respectively. The band broadening retains the bandwidth to be overlapped after implant positions change, leading to an enhancement in insensitivity to the implant position.