A low-profile high-directivity circularly polarized patch antenna is proposed in this article. Two orthogonal slots are etched out in the middle of a square patch. As a result, radiation patterns of original TM12/21 modes are reshaped to the desired ones that are similar to those of TM10/01 modes, leading to significant enhancement of broadside radiation. To accommodate the modified current distributions of the two operating modes, a pair of perturbed stubs is proposed and then loaded on the slot-loaded patch, which leads to the quadrature-phase difference. In order to suppress unwanted modes that tilt the radiation pattern away from broadside direction, differential-fed configuration instead of single-fed one is adopted to excite modified TM12/21 modes. Characteristic modes theory (CMT) is used to analyze the input impedance of differential-fed patch antenna and determine the two feeding positions. Next, the proposed antenna is compared with three conventional ones, which reveals its superiority in the highest directivity. Besides, it is also discussed how to determine the size of the finite ground plane. We find that the size of 1.5λ0 × 1.5λ0 can increase directivity slightly and effectively prevent radiation pattern from tilting tendency. Finally, a prototype antenna is fabricated and measured for validation.