According to national and international highly competitive environment, organizations must increase the outsourcing level of their activities in order to focus on their main capabilities. This will increase their dependency to performance of suppliers. Hence, organizations aiming in presenting high quality and low price products cannot overlook the performance of their elements. Therefore, activities of an organization in order to assess and select efficient suppliers lead to competitive advantage and superior performance of that organization. Data Envelope Analysis (DEA) is an efficient model when evaluating efficiency of homogenous units having several inputs and outputs. Also, problem of evaluation and selection of suppliers is a complicated decision making problem in real world cases in which there are various kinds of uncertainties. Grey number theory is utilized when facing uncertain situations. In this research, in order to evaluate and select efficient suppliers, two mathematical models using DEA and Grey number theory are proposed. Also novel ranking techniques proposed for such units which their efficiency has interval form. The first model is a developed form of Grey Data Envelopment Analysis (GDEA) which is obtained via considering decision makers priorities on inputs and outputs. In the second model,grey relational analysis and DEA are used in assessing and selecting efficient suppliers. According to small computational effort and less time consumption of the second model, this model is recommended for problems relating assessment and selection of efficient units. Also, in order to describe the performance of the proposed ranking technique, in addition to use of this method in solving examples for the first and the second model, a different example has been considered and the final results are compared with two other existing ranking techniques. Keywords: Evaluation and Selection of Efficient Suppliers, Data Envelopment Analysis (DEA), Grey Number Theory, Grey Relational Analysis.