This paper discusses about the design of circular patch microstrip antenna for Radio Frequency Identification (RFID) tags application in the microwave band. Circular microstrip patch antenna is designed with a rectangular slot with the addition of 45° rotation in antenna patch and feed line to use for electric current feed. The dimensions of microstrip antenna obtained through computation, and then performed simulations. The design of microstrip antenna using fiberglass epoxy material -FR-4 with dielectric constant (εr) = 4,5. Based on simulation results, the antenna shows the maximum performance at 2400 MHz with return loss < −10 dB and VSWR < 2. Return loss and VSWR consecutive minimum of −31.37 dB and 1.056; the gain value is 1.399 dBi with 83.4 MHz of bandwidth and the shape of the radiation pattern of the antenna is directional.