High-resolution and three-dimensional X-ray imaging is becoming more and more popular. An X-ray microtomographic system with a CMOS image sensor has been developed. We have developed a novel area X-ray detector with a cost-effective CMOS image sensor. The sensing area of the detector is 55 x 55 mm(2) and the light from the phosphor screen is collected to the CMOS image sensor by a carefully designed optical system. Six lenses are assembled to reduce the radiation damage effect and increase the resolution and sensitivity. A microfocus X-ray tube that can reach up to 5 mum of focal spot size and microprecision motor system that can move in x-y-z directions for both alignment and magnification and rotate the object have been adopted. A conventional three-dimensional cone-beam Feldkamp reconstruction algorithm was used and a human cancellous; bone has been imaged with this system. Currently, the resolution of this system is about 40 mum but the application in microtomography seems very promising using this CMOS X-ray detector.