Although the unique properties of CVD diamond films have made it a candidate material for radiation detectors, the detector performance is strongly dependent on the film quality. In this paper, three CVD diamond films with different grain size were grown by using a hot-filament chemical vapor deposition (HFCVD) technique and the ratio of the grain size to the film thickness is high to 50%. 5.9 keV Fe-55 X-rays measured the photocurrents and the pulse height distributions (PHDs) of these CVD diamond detectors. The detector performance is improved with the grain size increasing. The dark-current of 16.0 nA and the photocurrent of 15.9 nA are obtained at an electrical field of 50 kV(.)cm(-1) and the PHD peak is well separated from the noise pedestal. (c) 2005 Springer Science + Business Media, Inc.