This research was aimed to study the synthesis of cellulose nanofibers from Sesbania Javanica and the effect of the cellulose nanofiber (CNF) addition on thermoplastic starch derived from tapioca flour. The mechanical properties, morphological structure and thermal stability of neat thermoplastic starch and their composites were investigated by Tensile testing, Scanning Electron Microscope (SEM) and Thermogravimetric analysis (TGA), respectively. Moreover moisture absorption was also studied. The composites with 0, 1, 2 and 3 phr of cellulose nanofibers content were prepared. Cellulose nanofibers were synthesized in the aqueous suspension (containing 8.63% of dried cellulose nanofibers). The percentage yield was found of 14.8% and the average diameter is about 100 nm. The addition of cellulose nanofibers in thermoplastic starch showed that the modulus of the composites tended to improve, however, the tensile strength did not change significantly and tended to decrease when higher cellulose nanofibers content. SEM revealed the formation of CNF aggregates in the composites, especially the content of 3.0 phr of CNF, that is why no improvement in the tensile strength of the composites. Also, the moisture absorption of the TPS/CNF composite improved with the cellulose nanofibers content.