This paper presents a complementary metal-oxide-semiconductor (CMOS) compatible urea enzyme field effect transistor (FET) without enzyme immobilization. The natural formed aluminum oxide (Al2O3) above the top metal is used as the hydrogen sensing membrane. All the devices were fabricated by TSMC 0.35μm 2P4M CMOS process. In order to realize the micro sensing biological sensing system, the traditional Ag/AgCl reference electrode is replaced by the pseudo Ag/AgCl reference electrode in this study. First, the pH-FET is measured with the pseudo Ag/AgCl reference electrode. The sensitivity is 52.7mV/pH. Then, the droplet contains urea, urease, and Tris buffer solution was adopted above the sensing area. It gives a linear response when the urea concentration is varied from 1∼11mM. And it shows a slope of 88.7mV/mM. This result demonstrates that a CMOS compatible urea biosensor could be realized without immobilizing the urease above the sensing area.