The objective of this study is to characterize the quantum efficiency of a digital x-ray imaging system. This system is designed for small animal experiments. It is equipped with an x-ray tube with a Be-filtered tungsten target, 0.02 mm focal spot and 0.3 mA operating current. The fiber optically coupled CCD module, through a scintillating screen, covers a field of 5 cm 10 cm, at 1024 2048 pixel format. To analyze the impact of x-ray photon flux and CCD electronic noise to the quantum efficiency of the system, the noise power spectrum (NPS) and detective quantum efficiency (DQE) were measured as a function of x-ray exposure and detector integration time. A BR-12 phantom was placed between the x-ray tube and the detector during measurements. The results showed consistent DQE when exposure/integration is in the range of 2 to 7 seconds (for a 0.5cm thick phantom), and 3 to 15 seconds (for a 2cm thick phantom). With a 0.5cm phantom, DQE are approximately 36.7%, 25% and 5.4% at frequencies of 0 lp/mm, 3 lp/mm and 8 lp/mm respectively. With a 2 cm BR-12 slab, hardened x-ray beam at 26 KVp doesn’t have much impact on DQE, approximately 36.2%, 27.3% and 6% for 0 lp/mm, 3 lp/mm and 8 lp/mm frequencies. In summary, the CCD based digital x-ray imaging system investigated in this study is an efficient, x-ray quantum noise limited system for small animal experiments.