This paper presents a new method to rank nodes in power systems in terms of how close the loads at those nodes are to the limit that the system can supply before voltage collapse occurs. This method is applicable to slow load fluctuation and static modeling is used. The method is based on the fact that the Jacobian Matrix becomes singular at the voltage collapse point. This paper focuses on calculating the maximum load margin at load buses before reaching voltage collapse gradually (static voltage instability). After get the limit of bus load, the load bus could be ranked.