The polycrystalline nature of the films has limited the improvement of CVD diamond radiation detectors. In this work, three CVD diamond films with different microstructures were grown by a hot-filament chemical vapor deposition (HFCVD) technique and were fabricated as CVD diamond radiation detectors. The photocurrents and the pulse height distributions (PHDs) obtained by the detectors were measured by using 5.9 keV 55 Fe X-rays. The dark-current of 16.0 nA and the photocurrent of 15.9 nA are achieved at a bias voltage of 100 V. The PHD peak is well separated from the noise and a better energy resolution of 21.2% is obtained due to the high ratio of grain size to the film thickness. The time-dependent photocurrent initially increases rapidly and then levels off due to the 'polarization' effect. (c) 2005 Elsevier B.V. All rights reserved.