The demand of fresh water is increasing twice as fast as population growth, while the natural resources remain invariable or in reduction. Otherwise, the needs of development make that the demand will be superior to resources. To satisfy these last, new techniques of drinking water production implementation by using the desalination of sea water by reverse osmosis. We have the results relating to the pretreatment of the sea water plant desalination by reverse osmosis located in (Corso) the east of Algiers at the Mediterranean sea side. With an aim of increasing the efficiency and the lifespan of reverse osmosis plant in order to avoid or to minimize, the scaling and the membranes fouling, the water pretreatment sea is necessary. The sea characteristics water are: total hardness (3907), conductivity (55,000 mu s/cm), turbidity (8.40 NTU). Like technique of pretreatment, we chose the water softening of sea by lime, the resin and ultrafiltration, and finally coupling ions exchange - ultrafiltration and coupling precipitation by lime-ultrafiltration. Three resins amberlite CG 50, CG 400, IR 120 were used. The ultrafitration tests have been made on tubular mineral membrane CARBOSEP M2 (15 kg/mol). The coupling treatment of the filtrate by ions exchange (amberlite IR 120) to ultrafiltration gave the best result with 98% of turbidity reduction, a final value of 0.13 NTU. An improvement of the limit permeate flux was obtained, where the value passed from 30.6 L/h m(2) (ultrafiltration alone) to a value of 47.5 L/h m(2) for a concentration of 360 mg/L of the resin amberlite IR 120 with an increase of 35%.