This article presents the design and analysis of dual-band implantable antennas based on effective relative permittivity calculation. First, the effective relative permittivity for a microstrip antenna inside lossy tissue is analyzed, helping establish the basis for the design of our proposed dual-band implantable antenna. The antenna operates in 1.4 GHz Wireless Medical Telemetry Service (WMTS) band and 2.45 GHz industrial, scientific, and medical (ISM) band under proper feeding scheme. The lower band with quarter-wavelength resonance is realized by introducing two shorting pins to orthogonal strips. As a disturbance between two arc-shaped tails with open end is introduced, it can achieve circular polarization (CP) radiation property. Meanwhile, the higher band with half-wavelength resonance is realized through the path connecting two asymmetric open-end tails, so as to achieve linear polarization (LP) radiation property. As such, a wide CP bandwidth of 10.38% in WMTS band and a wide impedance bandwidth of 21.3% in ISM band are achieved and in final verified in experiment.