Abstract: Based on the friction equilibrium equation, the turbulent exchange coefficient K(z) is derived. Then, its finite difference form is used to compute local K(z) in the Ekman layer, supported by wind data from a double-theodolite. The following is the findings:(1) The K(z) is marked with daily variations of double peaks and troughs. Its average fluctuation can be four times as large as itself for the mean of the whole Ekman layer.(2) K undergoes diurnal variations consistent with mean tendency in temperature and potential temperature.(3) K corresponds to fluctuation in wind velocity. A point to note is that the mean K and the velocity vary in the same way at night, but exactly the opposite in daytime.An objective picture is thereby given for the daily variation intensity in mixed turbulent exchange.