Effects of half delta-wing tabs on mixing and diffusion in an axisymmetric jet are experimentally investigated. The tabs are installed at the jet exit circumferentially with an equal interval. The number of the tab is varied from 1 to 6. The jet Reynolds number is set to 20,000. Instantaneous streamwise and radial (vertical or horizontal) velocities and temperature are mainly measured by a composite probe which consists of an X-type hot-wire and an I-type cold-wire. The results show that mixing with the ambient fluid is more enhanced with increasing the number of the tab near the jet exit. However, in the downstream region, it is suppressed in the cases with 4, 5, and 6 tabs and most suppressed in the case with 6 tabs, while it is most enhanced in the case with 3 tabs. The analysis based on the mean velocity distribution indicates that, although the spatially-averaged entrainment velocity is basically decreased by the tabs, the mixing and entrainment can be enhanced when the entrainment area is significantly increased. Thermal diffusion is more enhanced with increasing the number of the tab near the jet exit but, in the downstream region, it is equivalent or suppressed in the cases with the tabs. These results are caused by the mixed effects of the tabs: enhancement of not only mixing and diffusion but also the energy dissipation. The present study suggests that it is feasible to both promote and suppress mixing of the jet by the half delta-wing tabs, and directional characteristics of the jet diffusion can be controlled by optimizing configuration of the tabs. (C) 2017 Elsevier Ltd. All rights reserved.