This thesis presents a low power transmitter front-end for 915MHz band IEEE 802.15.4 ZigBee applications. In this thesis the consideration issues in wireless transmitter have been discussed as well as the circuit design detail. The up-conversion mixer with the LO leakage suppressed at the RF output circuit has been proposed. The conventional Gilbert cell based mixer adopted a DC negative feedback which suppresses the DC offset between the different branches of the mixer. These DC mismatches in different branches of the mixer has been proved to cause the LO leakage at the RF output. Using simulation and some mathematic analysis, by removing these DC mismatches, the LO leakage at the RF output is suppressed. The analysis has been proved by the fact that the LO leakage at the RF output was more suppressed when deeply remove the DC offset at the baseband of the transmitter. The experimental results have been also presented to prove the theory. The transmitter front-end was designed and fabricated in 0.18$\mum$ CMOS technology. The transmitter front-end shows 13 dB power gain, 12 dBm of the OIP3, 0 dBm of the OP1dB. The transmitter front-end dissipates 10 mA from the 1.8V supply voltage. In the worst case the LO leakage at the antenna is -20 dBm when normally it is kept at -27 dBm. And by deeply removing the DC offset at the base band part, the LO leakage can be suppressed to more than -35 dBm.