The best capacity of fiber for different plant fiber composites and property index were obtained from experiment,and the best capacity of fiber corresponding to a single property index can be confirmed by the parabola equation. Furthermore, by combined the method of main composition analysis with the parabola equation, the best capacity of fiber can be determined accurately and reasonably which will provide the theorical basis and calculating method for determining the best capacity of plant fiber composites. The calculated optimum fibre contents of plant fibre composites are as follow:nylon matrix treated with coupling reaction 23.52%,nylon matrix treated with grafting reaction 23.77%,polypropylene matrix treated with coupling reaction 34.32%,polypropylene matrix treated with grafting reaction 27.71%.
