Unbleached and bleached Kraft cellulose pulp fibers modified with a long chain carboxylic acid, i.e. oleic acid in cold plasma conditions have been used as reinforcements in low density polyethylene (LDPE). The purpose of the modification is to enhance the interfacial adhesion between cellulose and matrix and to increase the dispersability. Composites containing up to 10wt.% of untreated and modified cellulose pulp fibers with LDPE were prepared by melt mixing. The samples were characterized by processing behavior, mechanical and rheological properties, SEM, contact angle measurements, TGA and DSC. It was found that when the modified pulp fibers were incorporated into composites matrix, most of the properties have been improved.