A FSK modulator for 9600 bps digital data transmission is designed and implemented by using PLL(Phase-Locked Loop) at UHF(435-440MHz) band and satisfies practical specification. This system consists of a voltage controlled oscillaltor, a reference crystal oscillator, two frequency dividers, a phase detector and a loop filter. For FSK modulation the modulation input is applied both at VCO and the reference oscillator. FSK modulated signal bandwidth is 15kHz. Output power is 11dBm with 12.5kHz side-lobe of -60.83dB. And 2nd harmonic component of output is -40.5dB.