The focus of this work was to use an inexpensive and easily available water-soluble natural resin like guar gum as a surface treating agent for jute yarns and to study their effect on the mechanical properties of the treated jute yarn reinforced vinylester resin matrix composites. Jute yarns were treated with 0.1, 0.2, and 0.3% guar gum solution at room temperature for 30 min. The composites were fabricated with the untreated and treated yarns and the vinylester resin having 36 wt % fiber loading. Maximum improvement was observed in the 0.2% guar-gum treated, jute yarn reinforced composites. The flexural modulus and flexural strength improved by 23.85 and 7.7%, respectively, for the 0.2% treated composites. The fracture behavior of the composites differed considerably, depending on the concentration of guar gum that was used. The experimental results gave a positive indication that simple ecofriendly chemicals like guar gum can be used effectively as surface treating agents of jute yarns for enhancement of the mechanical properties of the jute composites and can be a success from the technocommercial point of view. (c) 2005 Wiley Periodicals, Inc.