Aim: In alumina-on-alumina (Al/Al) THA, microsepa-ration is reported to accelerate wear and to be a risk factor of alumina cup fracture. While microseparation after THA is detected during walking, microseparation also occurs in relation to the hip position. Using Al/Al THA with cemented polyethylene-back alumina cup, the microseparation is clearly visible on plain roentgen-ograms. The aim of this study is to investigate the factors that affect hip position related microseparation. Materials and Methods: We performed 65 primary Al/Al THA. An average age at operation was 61.1 years and an average follow-up period was 29.5 months. Roentgenograms were taken in hip neutral position, abduction, adduction, external rotation and internal rotation. Results: Microseparation was observed in 15 hips (23.1%); 10 hips with hip external rotation and 5 hips with hip abduction. An average abduction angle of the cup was 33.5±9.8 degrees with microseparation and 36.6±7.3 degrees without microseparation. An average anteversion of the cup was 9.7±6.7 degrees and 7.8±5.8 degrees, respectively. The factors that brought about microseparation were postoperative muscle weakness in 3 hips, impingement in 9, cup malposition (abduction angle: 19, 23 degrees) in 2, and short off-set stem in one. Discussion and Conclusions: It should be noted that microseparation is not a rare phenomenon and it occurs in hip particular position even if it can not be detected on routine roentgenograms.