The establishment of rational balance between number of users and productivity capacity of rangelands and suitable number of the folk in the ranch units is one of the necessities of the sustainable management of rangelands. Also growth and efficiency of the productive factor would lead to the economical benefit maximum .This study was done in order to evaluate the optimal ranch size basis on the social and economical relative sustainability in the rangelands of Urmiya. The statistical information were derived from 203 questionnaire collected from 34 rangeland ranch units in the study area at the year of 2007-2008. Then the costs, incomes and grazing capacity were evaluated in each unit. The suitable number of folk and optimal ranch size were determined based on some criteria such as Total Efficiency Index. Productivity factors and annual costs of family were analyzed using Eviews economic software. The results showed that ranch size per a rancher is not optimal and each productivity unit has used from the rangeland less than optimal limit. At present, utilizable rangeland average per a user is 71 ha. Also the suitable minimum size of folk and rangeland per a family including 5 members is 550 and 350 ha, respectively. So it is able to supply the annual costs of family. Also economically suitable folk number and optimal ranch size were determined 667 and 297 ha, respectively. Therefore we can totally conclude that the folk size of 300 to 350 is a suitable size, either with respect to efficiency of production factors livelihood or preparing of the livelihood per a family unit, which completely depends on the ranching in the region.