Design approach to a novel balanced, circularly polarized (CP) square loop antenna under even-mode resonance is proposed in this paper. The loop antenna is diagonally fed by a fork-like dipole launcher. By matching the respective natural boundary condition of the loop radiator and the dipole launcher, the resonant even-mode with inherent CP radiation characteristic can be sufficiently excited. Both the bi- and unidirectional cases are designed and investigated. A bi-directional CP loop antenna with a 3-dB ratio axial ratio (AR) bandwidth of 6.6% is designed at first. A unidirectional antenna having a planar metallic reflector is further designed. The unidirectional loop antenna exhibits excellent uni-directional CP performance with a high front-to-back ratio of 40dB and a 3-dB AR bandwidth of 7.6% in both theory and experiment.