Banana fibers received four treatments, namely squeezing, alkali and hydrogen peroxide treatment, washing with water, and drying. Thermoplastic starch composites was prepared with glycerol as the plasticizer, banana fiber as reinforcement and thermoplastic cassava starch as matrix. The banana fiber could increase processing torque highly, while the effects of fiber contents on peak torque at 3min was so obvious. X-ray diffractograms illustrated that with increasing fiber content, cellulose crystallinity at 22.5° gradually got stronger but starch crystallinity at 20° almost didn’t. SEM micrographs showed good dispersion and adhesion between starch and fiber. Studies in the dependence of mechanical properties of reinforced TPS on the contents of fiber that with increasing fiber content from 0 to 20phr, the initial tensile strength was trebled up to 14.46 MPa, while the elongation at break was reduced from 68 to 13%. TG mass loss curves showed that thermal stability of this composites had great improved under 500°C.