Abstract: This paper presents a ring oscillator based fully integrated fractional-N frequency synthesizer suit for RF transceiver. MASH1-1-1 Σ-Δ modulation is implemented to randomize the fractional spur. Spectrum purity and power consumption is also considered during the circuit design. The synthesizer was implemented in TSMC 180 nm CMOS. The results indicate that 25 kHz frequency resolution could be achieved at a range of output frequency from 900 MHz to 1.4 GHz. When output frequency is 1.2 GHz, the phase noise is -106 dBc/Hz at 1 MHz offset and the settling time is 10 μs below.