A novel planar endfire circularly polarized (CP) antenna is presented and explored by using concentric annular sector complementary dipoles. The antenna is composed of a semicircular disk radiator operating at TM mode and a concentric annular sector dipole operating at one-half wavelength mode. A set of closed-form formulas is derived to reveal the operation principle and employed to determine the geometrical parameters of the proposed CP antenna. The design approach is both numerically and experimentally validated. Good agreement between the theoretical, numerical and experimental results has demonstrated its endfire radiation beam in parallel with its plane. The fractional impedance bandwidth for |S| < −10 dB is up to 6.45%, and axial ratio (AR) bandwidth of less than 3 dB is up to 14.43% in fraction, in addition to its simple geometry with only seven design parameters.