A method for exact evaluation of ligament integrity of the knee joint would be of clinical importance from the diagnostic point of view as well as from the desire to introduce objective measurement of stability either pre-and postoperatively, or before and after conservative treatment. Assessment of the posterior cruciate injury has been considered by numerous authors over the past decades. Even stress machine, such as Gonylaxometer, knee ligament arthrometer(KT-l000) and Genucome, have many problems such as soft tissue error, expansiveness and low popularity. Generally, orthopedic surgeons use passive tests, in which the displacing force is applied externally, to evaluate the integrity of the ligaments of the knee. Posterior tibial displacement of knee was measured at quadriceps neutral angle using a specially designed frame which minimized the rotational error to negate the quadriceps pulling effect. Posterior displacement of both sides of knee was measured in 15 patients who had posterior cruciate ligament injury and 40 adults who had normal knee. The following results were obtained. l. In the normal knee, the mean score of posterior tibial displacement was 3.0875mm in left, 2.8125mm in right each other and there was a mean of 1.05mm posterior displacement of tibia compared to the contralateral knee(range: 0–2.5mm). 2. More than 6mm displacement of lateral femoral condyle compared to the uninjured knee suggested posterior cruciate ligament injury. 3. This radiological measurement seems very efficient in detecting the posterior cruciate ligament instability of knee because of its simplicity, inexpensiveness and easy reproducibility with minimal error.