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We proposed a new design of CMOS-based X-ray image sensor with monolithically grown pixelated CsI(Tl) on photosensor area for securing the maximally achievable spatial resolution for a given sensitivity determined by the CsI(Tl) thickness at a certain X-ray energy. The test version of a CMOS image sensor (CIS) was designed and fabricated using AMIS 0.5 mum standard CMOS process. The chip includes an 128times128 CMOS active pixel sensor(APS) array with 50 mum pitch, a row/column decoder, a sample & hold circuit with buffers, an analog signal processor(ASP) and a 10 bit pipe-lined analog-to-digital converter(ADC). A data acquisition system (DAS) for operating the image sensor and for processing digital signals was also developed. Then the X-ray image sensor was fabricated by depositing thermally evaporated CsI(Tl) blocks of 50 mum thick on each pixel sensor of CIS using a photo-resist pattern as a basis of CsI(Tl) deposition. Linearity of the response, dark current noise, dynamic range and spatial resolution etc. were measured with bare and scintillator-coupled CIS image sensors by green LED light and X-ray beam respectively. The mean charge generation rate per pixel measured at room temperature in the dark was about 45,600 electrons/sec which results in the electronic noise of 210 electrons for 1 sec exposure time. Therefore the dynamic range is ~67 dB if we consider the statistical noise together. The spatial resolution of scintillator-coupled CMOS image sensor was measured to be ~7 lp/mm by a lead line pattern and 80 kVp X-ray. This works was supported by Nuclear R&D Program of MOST, Korea through KOSEF.