Recent progress in direct numerical simulations (DNS) of stratified turbulent flows has led to increasing attention to the applicability of the constant mixing coefficient Γ in the Osborn’s eddy diffusivity model, Γ = 0.2. Motivated by lack of observational estimates of Γ particularly in the deep ocean, this study examines variations of Γ using deep microstructure profiles collected in various regions of the North Pacific and the Southern Ocean. It is shown that Γ is not constant but varies significantly with the ratio of the Ozmidov scale to the Thorpe scale ROT in a fashion similar to that obtained by previous DNS studies. Efficient mixing events with Γ ~ O(1) and ROT ~ O(0.1) tend to be frequently observed in the deep ocean, while moderate mixing events with Γ ~ O(0.1) and ROT ~ O(1) tend to be observed in the upper ocean. Furthermore, the observed negative relationship between Γ and ROT is consistent with a simple scaling Γ proportional to ROT−4/3 that can be derived from classical turbulence theories. In contrast, the observed results exhibit no definite relationships between Γ and the buoyancy Reynolds number Reb, although Reb has long been thought to be another key parameter controlling Γ.