One of the best ways to decrease the costs in cold storage is to use the Optimum Insulation thickness (OIT) in external walls. Thus electricity usage is decreased by decreasing heat transfer occurred from wall and in consequence the costs decrease. Therefore the transmission loads, calculated by using measured long-term meteorological data for Afyon, are fed into an economic model (P1-P2 method) in order to determine the OIT. In this Study, It is showed the relation of the OIT, cost and saving with different storage temperature for a cold storage installed in Afyon (Turkey). Afyon is one of the coldest provinces in Turkey. The results suggest that rock wool is the most economical one among the other Insulation materials. For fiberglass, rock wool, extruded polystyrene and styrofoam, the OIT is respectively 0.071, 0.078, 0.047 and 0.067 in, and the saving occurs by using the OIT in external wall is respectively 42432, 44038, 40885 and 43272 $. Besides the OIT-decreases with temperature as linear, and the payback period increases with temperature.