Turbidity currents are one of the more frequently observed types of stratified flows. In these currents, the density difference is created as a result of suspended particles. The interfacial instability of turbidity current is studied experimentally in the present research. Both Kelvin–Helmholtz and (asymmetric) Holmboe instabilities are observed during the experiments; the first one was downstream, and the second one was upstream of the obstacle. Kelvin–Helmholtz instability is observed by approximately zero (phase) speed with respect to the mean flow. With the aim of measuring spectral distribution of velocity fluctuations, the effects of some parameters are studied on interfacial waves; Kelvin–Helmholtz waves weaken as the local Richardson number (J), an increase in inlet Froude number (Fin) or a reduction in obstacle height (H). The one-sided (asymmetry) Holmboe instability is detected moving to the upstream. This one-sidedness is as a result of asymmetry existence in velocity and density profiles in this research. Train of asymmetric Holmboe waves is observed while ejecting dense fluid into upper layer. The ejecting process happens randomly. Furthermore, secondary circulations behind these waves are perceived. Mentioned parameters (J, Fin, H) do not show meaningful effect on Holmboe waves. Phase speed of these waves increases with local Richardson number. The measured average wavelength of these waves is about 17 cm. The wavelength of one-sided Holmboe waves decreases as local Richardson number increases. The unstable region of one-sided Holmboe waves is obtained and is compared with linear stability’s prediction. © 2018, The Brazilian Society of Mechanical Sciences and Engineering.