A circularly-polarized (CP) slot antenna comprises of a triangular wide slot, a director patch, and a metallic reflector is proposed and studied in this paper. The CP radiation of the slot antenna is excited by the proximity coupling of an L-shaped strip. To obtain an optimum peak gain and a broad CP bandwidth, the effects of varying the positions of the director and reflector on the antenna performances are respectively investigated. Several prototypes were constructed, and the measured results show that the proposed antenna can provide an average gain of 10 dBi within a CP operating bandwidth of 23% and a front-to-back ratio of over 10 dB. Simulation analyses are also carried out to validate the experimental results.