Objective:To analyze the specific features of the pelvis osteolysis in cementless total hip arthroplasty and observe the differences between the pre-operative radiological evaluation and the true situation during operation. Methods:Six cementless hips of acetabulum loosening were revised in 2005. The osteolysis and defects were classified according to the method of Paprosky. Results:During the operations 1 case was type Ⅰ defects according to Paprosky which was evaluated non-osteolysis rediographically pre-operation. 3 cases were type Ⅱ defects which were diagnosed type Ⅰ pre-operation,and 2 cases of Ⅲ were diagnosed type Ⅱ pre-operation. 3 hips of type Ⅱ were revised with impacted bone graft and implantation of non-cement cups. 2 hips of type Ⅲ were revised with impacted bone graft and fixed with roof reinforced rings and cemented cups. 1 hip of type Ⅰ was revised with the fixation of a cemented cup within the original metal shell. All the patients were followed for 1 year radiographically and functionally. Conclusion:After cementless total hip arthroplasty,the osteolysis and bone defects in the acetabulum cannot be evaluated accurate radiography. More accurate methods should be used before operation,such as CT.