Operation principle and conceptual design of a non-uniform square loop circularly polarized (CP) antenna is proposed. At first, the even-mode square loop antenna with perimeter of one wavelength is analyzed by employing a simplified equivalent sources model. The inherent circularly polarized characteristic is theoretically revealed, and then used to conceptually design a non-uniform, bi-sensing square loop antenna at the 2.4GHz-band. It is found that the axial ratio (AR) of the loop antenna is greatly depending on the ratio of the width of the two adjacent edges. The proposed design approach should be promising to be used in the development of low structural complexity CP loop antennas.