Lignocellulosic fibers, as banana fiber, are potential substitutes for non-renewable synthetic fibers, nevertheless being a polar polymer, they have low compatibility with polyolefins, the most common non-polar thermoplastics. This difficulty can be minimized by chemical treatment of the fiber. In this work, HDPE composites and natural fibers were treated with two interfacial agents - stearic acid (AS) and lauric (AL). The interfacial agents and three types of fibers banana here defined as external Fiber (Fext), internal fiber (Fint) and intermediate fiber (Finterm). The interfacial agents were dissolved in acetone/toluene mixed with banana fibers at 50 °C for 24 h. Banana fibers, impregnated with the interfacial agents, were mixed with high density polyethylene (HDPE) in a twin screw extruder. The extruded samples were injected into specimens for evaluation of mechanical properties: tensile strength and impact strengths. The elastic moduli of treated fibers and HDPE composites increased as compared to pure HDPE. For the compound having HDPE and Fext-AL, the highest value for the modulus was found. This interfacial agent improved the elastic modulus of the material thus suggesting that this agent may have allowed a better interaction between the fiber and the olefinic polymer.