In situ polymerization by polycondensation was implemented to disperse crystalline nanocellulose (CNC) in poly(ethylene succinate) (PESu). The in situ polymerization technique resulted in good dispersion of the CNC nanoparticles in the PESu matrix. The polymerization kinetics were altered with increasing CNC content, causing a decrease in the polymer molecular weight. The addition of CNC particles having a large aspect ratio resulted in significant increases in the viscosity of the resulting composites, and significantly enhanced crystallization kinetics. PESu containing well-dispersed 0.25 wt% CNC maintained the tensile properties of the matrix, demonstrating that at these very low loadings the CNC acts as an efficient nucleating agent, without compromising the mechanical properties and biodegradability POLYM. ENG. SCI., 59:989-995, 2019. (c) 2019 Society of Plastics Engineers