Measurements of turbulent diffusivity in a plane mixing-layer flow with and without drop loading were conducted by a phase Doppler particle analyzer (PDPA). The mean free-stream velocity is 10 m/s, corresponding to a flow Reynolds number based on hydraulic diameter (0.15 m) of 95,600. Analysis of the data on mean velocity, Reynolds stresses, and turbulent velocity fluctuation reveals that the flow can be divided into two regions, the developing region and the fully developed region. In the developing region, the turbulent diffusivity is significantly reduced under two-phase condition except in tire initial region (i.e., X < 20 mm), where the turbulent diffusivity is higher under drop loading because the flow has been disturbed due to the existence of drops, This result is also consistent with that of Reynolds stresses and turbulent velocity fluctuation. However, the slope of the variation of turbulent diffusivity under drop loading is lower than that of the single-phase flow condition in this region because of the development of the large-scale structure in the flow. On the other hand, in the fully developed region, the decay of the turbulent diffusivity under the two-phase condition is lower than that of single-phase flow. Hence the energy dissipation due to viscous effects in the fully developed region has been partially compensated by the energy production due to turbulence modulation under the two-phase flow condition. This indicates that the modeling of turbulent diffusivity under the two-phase condition should take into account the turbulence modulation due to drop lending.