This thesis presents the analysis and countermeasure for image jitter in the gunner's primary sight of XK II. Vibration test was performed to identify the major jitter sources in the gunner's primary sight. The results showed that the image jitter was most significantly affected by two structural vibration modes, which were very sensitive to the mirror axis balancer in the gunner's primary sight. As a countermeasure for the image jitter, a rubber pad was inserted between the mirror axis balancer and its mount. The proposed countermeasure was very effective to reduce the image jitter by 70~90%.