This paper describes the design and implementation of a passive transponder chip for RFID applications. Passive transponders do not have their own power supply, and therefore all power required for the operation of a passive transponder must be drawn from the field of the reader. The designed transponder consists of a full wave rectifier to generate a dc supply voltage, a 128-bit mask ROM to store the information, and Manchester coding and load modulation circuits to be used for transmitting the information from the transponder to the reader. The transponder with a size 410 x 900 ${mu}$m$^2$ has been fabricated using 0.65 ${mu}$m 2-poly, 2-metal CMOS process. The measurement results show the data transmission rate of 3.9 kbps at RF frequency 125 kHz.