This paper describes an effect of fluctuation in fiber orientation on the tensile properties of sliver-based green composites. The composites were reinforced with slivers of high-strength natural fibers extracted from plants named curaua. Then a surface optical micrograph of the composites with the fiber fluctuation was obtained. The micrograph was divided into many fine segments, and the fiber orientation angle in each segment was measured. Results show that the tensile strength depends on autocorrelation coefficients expressing the degree of fluctuation in fiber orientation, as well as the fiber orientation angles. However, the Young’s modulus was dependent only on the angles, rather than on autocorrelation coefficients. In addition, a statistical concept was applied to an orthotropic analysis for prediction of the Young’s modulus. The predicted Young’s moduli showed better agreement with the experimental results.