This paper presents a new index to voltage collapse based on the system physical quantities such as nodes' voltages and injected powers.The index has a critical value. It is physically meaningful, varies smoothly when the system changes (i.e. does not exhibit discontinous changes on value for small system changes such as oLTCs´action). This new index does not need to build any models and not be limited with load patterns.Moreover it can be used to real time system monitoring because it can be easily calculated , and can alert operators to take appropriate actions. The simulate result is very promising.