Abstract: A chemical vapor deposition (CVD) diamond film detector with the diameter of 15 mm and thickness of 300 μm was fabricated for the Xray beam detection. The dark current of the detector is lower than 50 pA under the biased voltage of 800 V. The sensitivity and time response of the detector were investigated by both experiment and theoretic calculation. The experimental sensitivity is between 10-410-2 A·W-1 for 622 keV Xray, which is in good agreement with the theoretic results when the charge collection efficiency is assumed to be 39%. Resistancecapacity time constant is 1.5 ns. The pulse response rising time for subnanosecond pulse Xray is 23 ns.