A fully integrated direct up-conversion transmitter for Chinese electronic toll collection system (ETCS) is presented in a 0.18um CMOS process. Improved isolation between power amplifier (PA) and voltage-control oscillator (VCO) is achieved by configuring the VCO frequency to be 2/3 transmission frequency. A 5.8GHz mixer with an automatic amplitude control (AAC) loop is proposed to obtain better amplitude shift keying (ASK) performance. The occupied bandwidth of the transmitter is optimized by digital filtering. A high-linearity ASK modulator translates the base-band signal to carrier frequency and a two-stage class-A and class-AB PA is adopted to obtain sufficient efficiency and relatively high linearity. The transmitter consumes only 99mW with 1.8V supply voltage to meet the low-power demand of Chinese ETCS and carries a single-ended 3.2dBm output power. Under 1.024Mbps data rate, the eye-opening is better than 91% and the transmitter achieves -58.2dBc adjacent channel power ratio (ACPR) and 1.46MHz occupied bandwidth.