Debonding of cement-femoral stem interface has been suggested as a initial focus of loosening mechanism in many previous studies of cemented total hip replacement. The purpose of this study was to investigate the effect of debonding of cement-femoral stem interface to the cement-bone inter- face by using three-dimensional non-linear finite element analysis. Three cases of partial debonded, full debonded, and full bonded cement-bone interface were modelled with partial bonding of distal 70mm from the tip of femoral stem. Each situation was studied under loading simulating one-leg stanced gait of 68kg patient. The results showed that under partial and full debonded cement-stem interface conditions the peak von Mises stress (3.1 MPa) were observed at the cement of cement-bone interface just under the calcar of proximal medial of femur, and sudden high peak stresses (3.5 MPa) were developed at the distal tip of femoral stem at the lateral bone-cement interface in all 3 cases of bonding. The stresses were transfered very little to the cement of upper lateral bone-cement interface in partial and full debonded cases. Once partial or full debonded cement-femoral stem interface occured, 3 times higher stress concentration were developed on the cement of proximal medial cement-bone interface than full bonded interface, and these could cause loosening of cemented total hip replacement. Clinically, preservation of more rigid cement-femoral stem interface may be important factor to prevent loosening of femoral stem.