A bearings-only system is considered to be observable if, and only if, the target motion parameters can be uniquely determined by bearings-only observations. The problem of observability for bearings-only target location is discussed in this paper based on the target traveling in the three-dimensional space at a uniform velocity. The vector-graph between target and observer is drawn and vector equations are founded, by utilizing Gramme rule and rank of matrix, the observability of the observer with a uniform velocity and a uniform acceleration is obtained. It is shown that the target is unobservable toward observer with a uniform velocity as the observability index is a constant; the target is observable if the observability index is not a constant toward observer with a uniform acceleration. The study done in this paper is valuable to solving the bearings-only target observability analysis and it is of constructive sense to producing optimal trajectory of the observer.