Degradation of high frequency noise characteristics in 0.18 m NMOSFET induced by gate oxide breakdown has been characterized in the frequency range of 2 to 14 GHz. A serous degradation of microwave noise performance has been observed. The degradation mechanisms are analyzed by extraction of the channel and gate noise using a noise equivalent circuit model. It has been found that, gate shot noise, which is commonly ignored in the as-processed NMOSFET, plays a dominant role in determining the microwave noise performance in the post-oxide breakdown NMOSFET.