the importance of polyaniline and by modifying its properties with composites, additionally, considering the importance of starch in constructing deflectable polymers, it has been decided to construct a nanoscale from carboxymethyl starch by modifying the structure and methylation of the polymer. Due to their nanometer sizes, filler dispersion nanocomposites exhibit considerably improved properties when compared to the pure polymers or their traditional composites. Increased modulus and strength, outstanding barrier properties, improved solvent and heat resistance and decreased flammability are among the characterization techniques. Therefore, by using carboxymethyl starch (SCOOH), increasing polar properties and increasing these natural polymers in the polyaniline matrix, certain effects and results such as modification of thermal properties, solubility and the increase in water absorption in starch without changing or impairing properties of the polymer were observed, including thermal properties and conduction. These nanomaterials are widely employed in optoelectronics, electronics, medicine and photonics [3][4][5][6]. In addition to these interesting properties, the potential technological applications such as an active material in electrooptical devices, modulators and optical switches also can be found [7]. Third-order nonlinearity in conducting materials is extremely important parameter because of its various properties, including multi-photon absorption, free carrier absorption, nonlinear scattering and refraction [8,9].Nonlinear optical absorption and refraction of the nanocomposite were investigated in two different solvents using open and closed aperture Z-scan techniques, respectively. The synthesized sample has been characterized for its nanostructures by using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Also, the linear optical properties, including UV-Visible absorption, were measured.Abstract In this paper, the third-order nonlinear optical properties of modified nanocomposite carboxymethyl starch are presented. The nanocomposite of carboxymethyl starch has been synthesized by a chemical technique. X-ray diffraction and scanning electron microscopy were used to study its crystal structure. Linear optical response of this sample was studied by using UV-Visible spectroscopy. Nonlinear refraction and absorption coefficients of the nanocomposite were measured in two different solvents, dimethylformamide and N-methylpyrrolidone, by Z-scan method using a continuous wave Nd-YAG laser at 532 nm. The measured values of nonlinear refraction in both of the solutions were from the order of 10 −6 cm 2 /W.