An artificially two-dimensional metamaterial (ATDM) substrate is proposed as an artificial metamaterial high-constitutive parameter substrate for miniaturizing of a circularly-polarized microstrip antenna. In a circularly-polarized antenna, the electric and magnetic field directions are changing, which requires a two-dimensional metamaterial unit cell. The presented ATDM substrate raises the permeability and permittivity of the underneath substrate for a circularly-polarized patch antenna, and it is constructed of periodically arranged split-ring resonator (SRR) circuits implemented in a low permittivity dielectric underneath substrate. The ATDM attains equal permittivity and permeability material (ε r ∼ = μ r), which neutralizes the destructive effect of the increased permittivity on the bandwidth. In addition, the ATDM structure is implemented in printed circuit board technology. The area of the ATDM antenna at 2.45 GHz is approximately 75% smaller than a usual microstrip antenna. The proposed antenna bandwidth is enhanced compared to the antennas with high-permittivity substrates. The proposed ATDM substrate antenna is fabricated and measured, and comparisons show good agreements between simulated and measured results.