In situ energy dispersive X-ray diffraction measurements with synchrotron radiation source have been performed on nanocrystalline Ge with particle sizes 13, 49 and 100 nm by using diamond anvil cell. Whereas the percentage volume collapse at the transition is almost constant, the values of the bulk modulus and transition pressure increase with decreasing particle size. Models are suggested to describe the grain-size dependences of bulk modulus and transition pressure. The trend of the predicted results is consistent with the experimental results while the absolute values are still different from experimental ones.