Osmium is one of the chemical element s that in periodic table of elements is in the platinum group metals. This metal is one of the rarest elemnents in the earth crust and is found as a trace element in alloys, mostly i platinum ores. Osmium oxide in form of osmium tetroxide is a powerful oxidizer, which boils at 130° C . Osmium because of its high density is commonly known as the hardest metal.This metal within the platinum group metals has the highest melting point and lowest vapor pressure. The commo oxidation states of osmium are 2+, 3+, 4+, and 8+, but it can form compounds with oxidation states ranging from and 2- to 8 +. In this research, extraction and enrichment of osmium IV from dilute synthesized wastewater prepared by disolution of osmium tetroxide in hydrochloric acid, using nano-emulsion liquid membrane (NELM) technique is investigated. In emulsion liquid membrane (ELM) process, because of many advantages such as high mass transfer flux induced by high mass transfer area, presence of chemical reaction, and lack of equilibrium limitations has attracted much attention in recent deacades. Extraction of heavy metals from dilute solutions is one of the most important applications of this method. The surface actaive agent, carrier, membrane phase, and internal phase used in this study are: Span 80, ethyhexyl phosphoric acid, liquid paraffin, and hydrochloric aacid. Considering the large number of effective parameters and their interactions in this process, for optimization of the number of experiments, Taguchi method of design of experiments and Minitab 16 software package was used, and 25 experiments was performed to study extraction and enrichment of Os (IV) via NELM. The average size distribution of the nano emulsion using dynamic light scattering (DLS) was determined to be 64.3 nm. Effects of concentration of surface active agent, concentration of carrier, contact time, agitation speed, and volume ratio of membrane phase to feed on extraction and enrichment of Os (IV) were studied and optimum values for maximization of extraction and enrichment and minimization of emulsion swelling which are the responses of this system were determined.The optimum values for reaching the maximum extraction (98.2 %) based on design of experiments by Taguchi method are as follow. Concentration of surface active agent 2 % V/V, concentration of the carrier 3 % V/V, contact time 20 min, mixer speed 750 RPM, and volumetric ratio of membrane to feed 60 to 100. Effects of various parameters including concentration of surface active agent, emulsification time, concentration of the internal phase (stripping phase), and volumetric ratio of the internal phase to the oil phase on emulsion stability were studied, and the following optimum values were determined. Concentration of surface active agent 1.5 % V/V, time of emulsification 6 min, concentration of the internal phase 0.8 M, and volumetric ratio of the internal phase to the oil phase 1 to 1. Then, by performing real experiments, effects of concentration of surface active agent, concentration of the carrier, speed of agitator, contact time, volumetric ratio of membrane phase to feed, and concentration of the internal phase (stripping phase) on extraction and enrichment were studied. The optimal conditions for maximum extraction (99 %) were as follows: concentration of surface active agent 1.5 % V/V, concentration of V/V, contact time 20 min, speed of the agitator 750 RPM, concentration of the internal phase 0.8 M, and volumetric ratio of membrane to feed 30 to 100. Finally, extraction of Os (IV) from synthesized wastewater using a laboratory continuous emulsion liquid membrane column at mixing speeds of 200, 300, and 400 RPM under similar conditions with those of batch emulsion membrane was studied and compared. Results showed that extraction for continuous operation was 97 % compared with 88.5 % for batch operation.