Radiation detector of high performance was fabricated using free-standing thick CVD diamond films. The CVD diamond detector has “cross-finger” coplanar electrodes. The electrical contact between electrodes and diamond films is ohmic. The effective area is 3×3 mm2. When working under electric field strength of 30 kV/cm, the dark current of the detector is only about 01 nA, the rise time of the signal is about 590 ps, its sensitivity is about 110 mA/W. Besides, the detector has fairly good linearity on dose rate.