A 1.8V, 5GHz low power frequency synthesizer for Wireless Sensor Networks is presented in 0.18µm CMOS technology. A low power phase-switching prescaler is designed, and the current mode phase rotator is merged into the first divide-by-2 circuit of the prescaler to reduce power and propagation delay. An improved charge pump circuit is proposed to compensate for the dynamic effects with the charge pump. By a divide-by-2 circuit, the frequency synthesizer can provide a 2.324-2.714GHz quadrature output frequency in 1MHz steps with a 4MHz reference frequency. The measured output phase noise is -110dBc/Hz at 1-MHz offset frequency. The power consumption of the PLL is 11.2mW at 1.8V supply voltage.