The performance of Monin-Obukhov similarity (MOS) theory has been investigated for four types of events of transient physical processes in the stable nocturnal boundary layer during the Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99). These four types were internal gravity waves, Kelvin-Helmholtz (K-H) shear instability, a density current, and a low-level jet. It was found that the MOS theory generally holds whenever a new equilibrium is reached or recovered during or after these events but fails in the period of transition or the developing stage of the events with newly generated turbulence. Specifically, MOS was found to hold during the entire process of the nonbreaking internal gravity waves and during the well-developed stages both of the density current and of the low-level jet. It was found not valid during the process of the K-H instability and the developing stages of the density current. Characteristic features of these four types of events are also presented and analyzed.