A circularly polarized (CP) microstrip patch antenna with low profile and wide 3-dB axial ratio beamwiths (ARBWs) is presented in this letter. The broadening of 3-dB axial ratio beam (ARBW) mainly depends on the reduction of the electrical size of microstrip patch antenna at a certain frequency. Firstly, radial slots with a circular center and radial periphery are etched on the surface of the patch to reduce the size of the patch, and at the same time, the intensity of magnetic field excited by the slot is evenly distributed in the whole azimuth plane. Secondly, four rows of capacitive metallized blind holes are loaded around the metal floor opposite to the square radiation patch. The introduction of the capacitive substrate integrated impedance surface (CSIIS) further greatly reduces the size of the patch and broadens its axial ratio beam width. The measured results show that the -10-dB reflection coefficient bandwidth of this antenna is 2.27% (56 MHz, 2.433-2.489 GHz), and the 3-dB ARBW is 20 MHz (2.438-2.458 GHz). The 3-dB ARBWs of the antenna in two orthogonal main planes are 170° (φ = 0°) and 217° (φ = 90°) respectively, while the measured axial ratio beam widths (ARBWs) in several azimuth planes such as φ=30°, φ =45°, φ =60° and φ=135° are 189°, 180°, 151° and 198°, respectively.