Pain is common in patients after orthopedic surgery. The 11-face Faces Pain Scale has not been validated for use in adult patients with postoperative pain. To assess the validity of the 11-face Faces Pain Scale and its ability to detect responses to pain medications, and to determine whether the sensitivity of the 11-face Faces Pain Scale for detecting changes in pain intensity over time is associated with gender differences in adult postorthopedic surgery patients. The 11-face Faces Pain Scale was translated into Vietnamese using forward and back translation. Postoperative pain was assessed using an 11-point numerical rating scale and the 11-face Faces Pain Scale on the day of surgery, and before (Time 1) and every 30 minutes after (Times 2-5) the patients had taken pain medications on the first postoperative day. The 11-face Faces Pain Scale highly correlated with the numerical rating scale (r ¼ 0.78, p < .001). When the scores from each follow-up test (Times 2-5) were compared with those from the baseline test (Time 1), the effect sizes were L0.70, L1.05, L1.20, and L1.31, and the standardized response means were L1.17, L1.59, L1.66, and L1.82, respectively. The mean change in pain intensity, but not gender–time interaction effect, over the five time points was significant (F ¼ 182.03, p < .001). Our results support that the 11-face Faces Pain Scale is appropriate for measuring acute postoperative pain in adults.