Background: To observe micro lesions on the cartilage of the rabbit femoral head using phase-contrast imaging with synchrotron hard X-ray and to prove that this method can be useful in the study of the degeneration of cartilage. Methods: New Zealand white rabbits were used in a micro lesion model of rabbit femoral head cartilage. Bilateral femoral heads were excised from rabbits, and micro lesions were made on one side with a specially made knife with a blade 20 mu m in width. The other femur was left intact to serve as the control. Phase-contrast imaging with synchrotron hard X-ray and conventional X-ray imaging were used to observe the cartilage. Histological changes were investigated using modified Golden tri-color staining. Results: Phase-contrast imaging with synchrotron hard X-ray clearly showed the 20 mu m lesions on the cartilage on the heads of rabbit femurs. These lesions were not visible with conventional X-ray imaging. Histological observation confirmed the presence of the microscopic lesions. Conclusion: Phase-contrast imaging with synchrotron hard X-ray can detect microscopic lesions on cartilage that cannot be detected by conventional absorption-contrast X-ray. This provides an unequivocal, non-invasive alternative to histological examination in the diagnosis of joint disease. It should be considered a new tool in osteoarthritis and cartilage research.