This work aims at studying the preparation and characterization of composites of phenolic resin (matrix) based on cashew nut shell liquid, reinforced by natural jute fibers. The fibers were chemically modified using alkaline treatment with solutions of NaOH (5 and 10%) and bleached with sodium hypochlorite NaClO/H20 (1:1) at 60–75°C. The microstructure was investigated by Scanning Electron Microscopy to observe the fiber surface after the treatment. As a result, there was an improvement in the thermal stability of the fiber, which was verified by Thermogravimetric Analysis. The jute fiber composites showed an improvement in their mechanical properties due to chemical treatment with 5% NaOH. Their biodegradability level depended on the employed alkali solution concentration. This study is important to evaluate the application of the fibers as renewable materials. POLYM. COMPOS., 31:1928–1937, 2010. © 2010 Society of Plastics Engineers.