A 2.4GHz ASK transmitter suitable for low power wireless capsule endoscope system is presented. The mixer-based frequency up-conversion transmitter architecture is employed to achieve high data rate. A pseudo-differential stacked class-A power amplifier using the current reused technique is proposed to save power. The transmitter mainly includes two parts: a 20MHz ASK modulator based on the constant amplitude phase lock loop (PLL) and a direct up-conversion RF circuit. This design, implemented in TSMC 0.25mum CMOS process, achieves -23.217dBm output power with the data rate of 1Mbps and dissipates 3.17mA current from 2.5V power supply