The major cause of early dislocation after total hip arthroplasty is malposition of acetabular cup. Therefore, radiographic evaluation of acetabular cup placement is essential in determining the postsurgical prognosis. The purpose of this study was to establish principles which would aid in checking the radiography. We evaluated 33 patients who were treated with total hip arthroplasty between Jan. 1988 and Aug. 1993 and investigated the methods of measuring the position of the acetabular cup on radiography using pelvic skeleton and acetabular cup. The results were analyzed as followings. l. After the total hip arthroplasty, the inclination of the acetabular cup shows average 37.1 degress in pelvis AP, 36.4 degrees in hip AP and 43.4 degrees in femur AP. This result means that the more X-ray beam focus is far from the acetabular cup, the more inclination of acetabular cup on radiography is incerased. 2. After the total hip arthroplasty, the difference of acetabular cup angle between pelvis AP and hip AP is average 0.7 degree, 6.4 degrees between pelvis AP and femur AP and 7.1 degrees between hip AP and femur AP. This result means that the more X-ray beam focus is far from acetabular cup, the larger difference between acetabular cup angle on radiography is.3. In experimental study with pelvic rotation, acetabular cup angle on radiography is average 45.7 degrees in neutral position, 42.8 degrees in 10 degree rotation, and 39.6 degrees in 20 degree rotation. This result means that the more rotation of pelvic bone is increased, acetabular cup angle on radiography is reversely diminished. 4. In experimental study with pelvic flexion-extension, acetabular cup angle on radiography is average 45.7 degrees in neutral position, 56.4 degrees in 20 degree pelvic flexion, and 32.3 degrees in 20 degree pelvic extension. This result means that when the pelvis is flexed, acetabular cup angle on radiography is increased, and when the pelvis is extended, acetabular cup angle is decreased than normal acetabular cup angle. 5. In experimental study with X-ray beams, acetabular cup angle on radiography is average 45.7 degrees in pelvis AP, 45.5 degrees in femur AP and 50.4 degrees femur AP. This result means that the more X-ray beam focus is far from acetabular cup, the more inclination of acetabular cup on radiography is increased. As the result of the study, radiographs which focus centered on hip demonstrated a nearly true representation of acetabular cup angle.