Abstract: Diamond films with different grain sizes were prepared using the bias-enhanced hot filament chemical vapor deposition（HFCVD） technique.Atomic force microscopy（AFM） analysis shows that the films are composed of small diamond particles with grain size ranging from ~10μm to ~90nm and the roughness is from ~423nm to ~15nm.The I-V measurement results indicate that with reducing of the grain size,the resistivity of diamond films changes from ~10¹¹Ω·cm to ~10⁶Ω·cm.5.9 keV ~55 Fe X-rays are used to investigate the photocurrent and signal-to-noise ratio of the detectors based on diamond films at room temperature.The above results indicate that diamond films with larger grain size are more suitable for being fabricated as semiconductor detectors.