Advanced green composites having excellent strength and stiffness were fabricated using liquid crystalline (LC) cellulose fibers and soy protein isolate (SPI) resin. Further, LC cellulose fibers were treated with potassium hydroxide (KOH) to improve their tensile strength and Young's modulus by increase the crystallinity of cellulose. The improvements were significant when the treatment was carried out while keeping the fibers under tension. The Young's modulus (stiffness) of the LC cellulose fibers increased by about 33 % from 47.8 to 63.7 GPa and the strength increased by about 18 % from 1483 MPa to 1749 MPa. X-ray diffraction (XRD) study of the LC cellulose fibers showed over 50 % increase in crystallinity after the KOH treatment. The mechanical properties of the LC cellulose fiber-reinforced composites were also high and improved further when the KOH treated fibers were used. With 65 % fiber volume it should be possible to obtain composites with strength above 1020 MPa and modulus of over 37 GPa, making them truly advanced green composites that could be used for structural applications.