Purpose This study investigated the improvement of functional activity after primary total knee arthroplasty (TKA) to determine whether or not functional activity definitely improves with time. Methods The participants were 40 people suffering from knee osteoarthritics, who underwent primary TKA. The functional activity was measured using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) physical function subscale (17 items, Japanese version) preoperatively, at the time of discharge and after discharge. Results In all items, there was a significant improvement after discharge compared to preoperatively, and at the time of discharge compared to preoperatively. The following three items were significantly improved and one item was significantly worse after discharge compared to discharge time. Conclusions The functional activities that significantly improved after discharge compared to discharge time were activities that were not often performed during hospitalization. They were improved by performing them after discharge in real life scenarios and progress was shown with time.