Environmentally friendly green composites were fabricated from a natural cellulosic fiber (MAO fiber) and a biodegradable starch-based resin through hot-pressing. The effects of fiber length and alkali surface treatment on mechanical properties of the green composites were investigated. Fiber lengths of 2.5, 5, 10, and 20 mm were used and fiber weight content was adjusted to 56%, to obtain short fiber composites with random orientation. Ultimate tensile strength increased with increasing the fiber length up to 10 mm and remained almost constant for further increases in fiber length. Fracture strain for the composites fabricated with fiber length of 2.5 mm showed the smallest value of approximately 2 %, which is less than that of MAO fiber. This might be attributed to the debonding at the fiber/matrix interface. Fracture strains of the green composites with fibers longer than 2.5 mm were almost constant and were comparable to the fracture strains of MAO fiber indicating that the fracture properties were controlled by the fiber. Both tensile strength and Young’s modulus values were increased by alkali surface-treatment for MAO fibers. The reason for this behavior seems that alkali treatment increases the fiber/matrix interfacial adhesion strength primarily by removing lignin.