Hydraulic stability is an important criterion for evaluating hydronic networks. In this paper, a quantitative definition of hydraulic stability is presented, and the calculation method is introduced in detail. For a network, this criterion can be calculated according to the network's structure and hydraulic condition and it can also be calculated according to the measured data online. How helpful the value of the criterion is in understanding the performance of a network is discussed along with simulation of the control process. Finally, several typical networks--direct return network, reverse return network, and distributed pumps system--are presented as case studies.