The distribution of the Brunt-V?is?l? frequency is one of the most important dynamical characteristics of the ocean. In this paper the space-time variations of Brunt-V?is?l? frequency on the continential shelf of the East China Sea in summer are considered. It has?been confirmed that the state of sea water is related to ,the static stability frequency. In shallow sea as well .as in deep-sea, states of the depths for which the static stability is the largest are the most likely to be unstable dynamically. The vertical structure of sea water over the shelf of the East China Sea is also closely related to the static stability frequency. It is shown that 1) the number of water mass vertically may be determined by the number of the maximum of the vertical distrihution of mean Brunt-V?is?l? frequency with formula (3), 2) the depth :of the vertical boundary of water mass corresponds to that of the maximum stability, and 3) the depth of the core of intermediate water mass can be identified by the depth of the minimum stability.