Zero-phase-shift line (ZPSL) loop antennas have recently attracted much interest by virtue of their uniform magnetic field distribution and electrically large size, which make them particularly useful as reader antennas in ultra-high frequency (UHF) near-field radio frequency identification (RFID) systems. Nevertheless, the challenge to theoretically characterize this type of antenna has led to difficulties in optimizing the loop for an electrically large interrogation zone as an RFID reader antenna. In this paper, we present for the first time the design methodology of the ZPSL loop antennas by applying the dispersion characteristics of the ZPSL structures. The perimeter of the loop antenna can be increased up to 3λ0 without current reversal along the loop. Considering the tradeoff between the loop size and the strength of the magnetic field in the interrogation zone, a ZPSL loop prototype with a perimeter up to 2.5λ0 is designed and optimized to verify the modeling and optimization experimentally.