This paper describes a control of a moving target tracking for a mobile hand-eye system. Autonomous target tracking by vision system has many potential applications; e.g. surveillance, security, etc. In particular, mobile robot-based trackers arc attractive because they can track targets in wide range. However, mobile robots with wheeled systems cannot instantaneously move in the axel direction because of nonholonomic constraints. Therefore, a visual sensor mounted on a mobile robot may lose sight of an observed target when their relative motion is large. On the other hand, mobile hand-eye systems with extra d.o.f has great possibilities to track target objects due to redundancy. But there have been rarely previous researches in which effective utilization of the redundancy was considered. Hence. "Traekabilily measure" which is the index to evaluate tracking ability of a mobile hand-eye system is proposed in this paper. Not only operabilily of a hand-eye, but also mobility of a mobile robot and the a position of a target object are introduced as points of the index. The trackability measure makes it possible to track a moving target taking full advantage of a mobile handeye system.