This letter presents a novel single-fed microstrip patch antenna using three-dimensional (3-D) printed substrate for both circular polarization and wide 3 dB axial-ratio beamwidth (ARBW). Here, the substrate consisting of a photosensitive resin and air with a square mesh-grid structure is dual-function. On the one hand, the permittivity of the substrate in two orthogonal directions is tuned to be slightly different by inserting two resin chunks. Hence, the asymmetry of the substrate can split the two degenerate modes of the patch antenna so as to generate circularly polarized waves. On the other hand, the effective permittivity of the substrate can be freely controlled by the volume fraction of air, and thus influences the 3 dB ARBW. As a result, wide ARBW can be realized by properly configuring substrate through a 3-D printing process. A prototype is fabricated andmeasured. Measured results are found in reasonably good agreement with the simulated ones, exhibiting a 3 dB ARBW as wide as 120 degrees.