In the electricity market environment， adequate reactive power support service has to be procured so as to facilitate active power transactions and maintain the system security. Reactive power produced by differently located suppliers may have different effects on the system economics and security， and hence it is necessary to evaluate the value of reactive power support service so as to provide a solid foundation for reactive power pricing and guidance for reactive power planning and dispatching. Existing research work on this subject is mainly based on a deterministic operating scenario and cannot properly reflect the values of reactive power resources when many plausible system operation scenarios may occur. In order to make a comprehensive valuation for reactive power support service for such cases， a method based on voltage sensitivity is proposed from the perspective of voltage regulation. The IEEE 14-bus system is used to illustrate the proposed method， and some interesting results are observed and discussed.