Nanocrystals cellulose (CNC) has been used in studies as biotemplate to improve the electronic properties of conjugated polymers (CP) increasing their molecular orientation. This study demonstrates the in situ grafting of the CNC surface esterified with 3-thiophenetic acid (3TAA) during the oxidative polymerization of 3-hexylthiophene. The surface modification of the CNC was evidenced by the presence of the band with an absorption peak at 1737 cm−1 in the infrared region and the chemical displacement at 157 ppm of the 13C NMR, assigned to ester group carbon. The influence of cellulose nanocrystals in the molecular organization of the CP was studied by Ultraviolet-visible Spectroscopy (UV–vis) and Cyclic Voltammetry (CV). The lower band gap energy and anodic peak potential observed for the nanocomposite elaborated with 10 wt% CNC3TAA suggest the influence of nanocrystals on the increase in planarity of P3HT macromolecular chains, decreasing the energy expenditure in the oxidative process of the conjugated polymer. •Acid 3-thiophenoacetic acid be employed for surface modification of cellulose nanocrystals.•CNC are rigid fibers that can stimulate the molecular ordering of conjugated polymers.•CNC3TAA-g-P3HT showed an 8% decrease in band gap energy versus pure P3HT.