Abstarct: Various properties of rare elements caused increasing demand in different and new technologies and their prices in recent years. The fact is that their similar behavior in separation makes their differential separation very difficult. In recent years, a mixture of two organic solvents was used for better separation of rare earths from each other in solvent extraction method. Previous researches revealed that a number of these mixture systems could improve the differential separation of them. The aim of this research work and thesis is to find the possibility of differential separation of heavy and light rare earth elements using mixture systems i.e., Cyanex272 and HQ, Cyanex272 and D2EHPA, and finally HQ and D2EHPA. The results of different tests in this study showed that a mixture system of Cyanex272 and D2EHPA with volume ratio of 4 to 1 had the best fractional separation of rare earth elements including dysprosium, gadolinium, neodymium and lanthanum. The extraction efficiency of Dysprosium, gadolinium, neodymium and lanthanum using the mixture system of Cyanex272 and D2EHPA resulted in 99.92, 64.54, 26.47 and 30.07 percent, respectively. When lactic acid was used as a complexing agent in the the mixture system, the results showed that the fractional separation of rare earth elements were improved baxsed on the parameters of extraction efficiency and separation factor. The extraction efficiency of Dysprosium, gadolinium, neodymium and lanthanum in the mixture system using 0.8M lactic acid resulted in 94.03, 50.50, 2.90 and 1.00 percent, respectively. Stripping of this mixture system using lactic acid was carried out with dilute nitric acid. Striping efficiencies of Dysprosium, gadolinium, neodymium and lanthanum using mixture system of Cyanex272 and D2EHP together with 0.2 (ml) of 0.8 M lactic acid with 0. 1 M nitric acid resulted in 99.10, 91.78, 22.48 and 3.85 percent, respectively. After determining the best mixture system of solvents i.e., Cyanex272 and D2EHPA and complexing agent of lactic acid, the effective factors and optimum conditions for the separation of heavy from light rare earth elements were investigated using response surface methodology. The results showed that the most influential parameters in the separation of rare elements were pH, concentration of organic solvents, volumes of organic solvents, concentration of the complexing agent and time, respectively. The Optimization study showed that the optimum condition was at pH (0.1), the concentration of organic solvents (0.15 M), time (5 minutes) , the volume ratio of Cyanex272 to D2EHPA (1) and lactic acid concentration of (0.4M). At the optimum point separation of heavy and light rare earth elements with extraction efficiencies for Dysprosium (93.78), gadolinium (79.65), neodymium (7.85) and lanthanum (2.23) was obtained. The differential separation of heavy mextals (Dysprosium and gadolinium) were carried out under conditions of pH (1.1), the concentration of organic solvents (0.15 M), time (15 minutes), volume ratio of Cyanex272 to D2EHPA (4) and the concentration of lactic acid (0.4 M) . Extraction efficiencies of Dysprosium, gadolinium, neodymium and lanthanum, under above conditions, were obtained, 85.23, 42.27, 1.67 and 0.60, respectively. On the other hand, the separation of light rare elements (lanthanum and neodymium) were conducted under the conditions of the pH (2.2), the concentration of organic solvent (0.05 M), time (5.0 minutes), volume ratio of Cyanex272 to D2EHPA (2.5) and 0.4 M concentration of lactic acid, so that the extraction efficiencies of Dysprosium, gadolinium, neodymium and lanthanum, were obtained, 99.99, 99.00, 73.70 and 21.23 percent, respectively.