An X-ray pixel detector with photon counting technique for digital X-ray imaging was designed and developed. Si detector was fabricated starting from 5 in., FZ-refined, 620 mu m-thick,  oriented, n-typed silicon wafer with high resistivity of 6000-12,000 Omega cm. Readout front-end, which consists of the preamplifier, comparator, and bias circuits including the band-gap reference circuits, was designed and fabricated using 0.25 mu m-triple-well CMOS standard process. In detector, the several types of guard-ring structures were tested. The biased p-type guard ring showed more reasonable results in the leakage current and breakdown voltage. The experimental results for the readout chip prove that its functionality is correctly operated up to 100 mV, 2.5 M events/s. In radiation experiment under irradiation of Co-60 at dose rate 10 krad/h the measurement indicate that the band gap reference generator (BGR) circuits work up to 240 krad and the maximum variation of output voltage is 0.4% (peak-to-peak) of operational voltage at the range of 0-240 krad. It cannot lead to any critical problem for use in its operation. (C) 2007 Elsevier B.V. All rights reserved.