In cruciate-retaining (CR) type TKA, the increase in posterior condylar offset (PCO) is considered to be correlated to flexion angle acquired postoperatively according to the article reported by Bellemans (J Bone Joint Surg Br 84:50–53, 2002). However, the significance of PCO seems to differ according to the size of joints. We therefore have defined a new parameter of posterior condylar offset ratio (PCOR) on the lateral view of plain X-ray photographs and studied the relationship between PCOR and postoperative flexion status in posterior-stabilized (PS) type TKA. Flexion status includes two parameters, such as postoperative flexion angle (FA) and flexion achievement rate (AR). The subjects of this study were 160 knees (16 males and 144 females, average 75 years.) with PS type TKA for osteoarthritic knees between 1999 and 2003 at our institution, more than at least 1 year postoperative follow-up. In the study of FA, patients with FA of less than 100° were divided into Group L (n = 28), patients with FA of 130° and greater were divided into Group H (n = 58). In the study of AR, patients with AR of less than 100% were divided into Group P (n = 46), patients with AR of 120% and greater were divided into Group G (n = 22). PCOR was statistically compared in each group, respectively. In FA, PCOR in Group L (0.385) was significantly lower (P = 0.027) than that in Group H (0.428). In AR, PCOR in Group P (0.376) was significantly lower (P = 0.0018) than that in Group G (0.456). We have concluded, though there are many factors influencing the range of movement after TKA, our newly defined PCOR could possibly serve as a parameter of postoperative flexion status of PS type TKA on plain X-ray photographs.