We measured normal range and mean value of the posterior laxity of normal knee which might be reference for objective and accurate diagnosis, decision of operative indication and position of knee during fixation of ligament, and role of quadriceps building in posterior cruciate ligament injury patients. These results were obtained by measurement and analysis of posterior laxity of normal knee by means of KT-2000 Knee Ligament Arthrometer. We measured 184 Korean adults (100 males, 84 females), and their average age, height, weight, and thigh circumference were 29.8 years, 165.8cm, 59.8kg, 41.2cm, respectively. At 25 degrees of knee flexion, mean posterior displacement of normal knee was 21.2mm with 15lb posterior tibial loading, 2.41mm with 20lb posterior tibial loading. And when quadriceps were tense, laxity was 0.79mm and 0.95mm at the same degree of knee flexion. At 75 degrees of knee flexion, the results were 0.57mm, 0.58mm, 0.26mm, and 0.28mm respectively. Right to left difference of posterior displacement with 20lb posterior tibial loading was 0.07mm at 25 degrees of knee flexion and 0.06mm at 75 degrees of knee flexion. We thought that above 75 degrees of knee flexion is suitable for knee position at the time of graft fixation because 75 degrees of knee flexion markedly decreased(75%) posterior displacement compare to 25 degrees of knee flexion. When quadriceps were tense, remarkable decrease of posterior laxity of normal knee was checked(58%), so we noticed quadriceps building is important to management of posterior cruciate ligament injury patients.