In this paper, described is an empirical study on a display lag effects in an augmented reality environment using video-see-through techniques. The lag effects on two tasks, which manipulate real world objects, are actually studied. One task is designed for mainly using visual feed back from a video-see-through HMD. The other task is designed for using both visual and tactile feed back. In the former task's case, the task completion time are affected, when display lag is greater than 100 millisecond. However, in the latter taak case, there was no significant difference in the task completion times over the lag up to 166 millisecond. Through out the experiments, it is shown that, when a display lag is smaller that 100 millisecond, a video-see-through techniques can be used for realizing an augmented reality system with better registrations.