Research has indicated that knowledge of results (KR) contributes to the processes of motor skill learning. Considering the demographical changes in our society, facilitating seniors' motor skill acquisition is important for achieving an active life style, longevity, independency, and quality of life. However, the effects of KR on motor skill learning in aging populations are uncertain. Specifically, it is unclear if the post-KP interval would affect the behavioral characteristics of motor skill acquisition for seniors. Thus, the purpose of this study was to examine the effects of three post-KR intervals (3, 6, or 12 seconds) on skill learning of a motor timing task in seniors. Twenty-five senior participants (70.68+2.2 yrs) and 23 young adults (23.28 + 2.68) were requested to perform an arm movement task by moving a handle on a linear-slide apparatus. In the acquisition phase, KR was provided (timing error in ms) 3 seconds after each trial. Based on a counter-balanced order, the performer was requested to perform the task again at 3, 6, or 12 seconds (12 practice trials for each interval). In the retention phase, each learner performed 5 trials without the KR. The arm movement performance was measured in timing error (ms) relative to the standard time (1000 ms) at acquisition and retention phases. The data were analyzed by using ANOVAs repeated measures. The results indicated that in the acquisition phase, there were no significant differences between the older seniors (76.87+ 2.33 yrs, n=15; error = 368 ms) and the younger seniors (65.83+ 2.36, n=10; error = 398 ms), F (1, 23) = 2.18, p > .05. However, seniors performed less accurately than the young adults, F (2, 45) = 6.18, p < .01, suggesting that in general, aging results in poor motor performance and learning. In addition, the seniors in both age groups performed better at the shorter post-KR interval (3 and 6 seconds, error = 268 and 279 ms, respectively) than the longer post-KR interval (12 seconds, error = 388 ms), F (2, 23) = 5.38, p < .01. The findings suggest that post-KR intervals affect seniors' motor skill learning. Seniors would gain less from longer post-KR intervals (e.g., 12 seconds) than the shorter intervals (3 and 6 seconds). This could be explained by seniors' cognitive deficits or deterioration in memory or attention. Post-KR intervals appeared to be important factors in designing motor learning programs for senior citizens.Keyword(s): older adult/aging issues, physical activity, research