The Yellow Sea Cold Water Mass (YSCWM), which seasonally presents and occupies the central bottom trough, is the dominant hydrological phenomenon in YS in summer. YSCWM is considered to originate locally from previous winter water. The seasonal evolution of YSCWM has been revealed as growth in spring, maturity in summer, decline in autumn, and disappearance in winter. Cold water mass (CWM) in the North Yellow Sea (NYS) is of great significance to local hydrodynamics, marine ecosystems, fisheries, and especially aquiculture in the local marine farms. Using seabed-mounted TD/CTD chains and buoys, the temperature and current variation in the NYS Cold Water Mass (NYSCWM) was studied during the summer of 2009. Besides the flood-ebb and spring-neap variation, the near-inertial oscillation (NIO) was also found in this region, which was generated by the sudden change of winds. Long-term observation indicates that the NIO can only happen in summer due to the CWM generated strong stratification in the northern NYS. However, the stratification is too strong in the CWM occupied water that little energy (<10% of the wind work) can be transfer downward to the CWM interior. Most of the energy is dissipated at the base of mixed layer to drop down the mixed layer depth. Different from deep water, shear instability might be the dominate mechanism on decaying of NIO at mixed layer in the northern NYS.