The formation of a diurnal thermocline in the ocean mixed layer under a stabilizing buoyancy flux was simulated successfully by large-eddy simulation, reproducing various features consistent with observation. The analysis of the simulation result revealed that the formation of a diurnal thermocline passes through two different phases: the formation of a thermocline (formation stage) and increasing thickness of the thermocline thereafter (growth stage). Turbulent kinetic energy (TKE) flux dominates TKE production within the mixed layer, but turbulence maintained by shear production at the thermocline causes stratification below the mixed layer. In addition, once the thermocline is formed, both the gradient and flux Richardson numbers maintain constant values at the thermocline. It was also found that a diurnal thermocline cannot be formed in the absence of both wave breaking and Langmuir circulation. Furthermore, the effects of stratification on turbulence were investigated based on the time series of various physical variables of turbulence at the diurnal thermocline and within the mixed layer, and the mechanism for diurnal thermocline formation is discussed. Copyright 2009 American Meteorological Society