Polyethersulfone (PES) and PES/cellulose nanocrystal (CNC) nanocomposite fibers were prepared via dry-jet wet spinning at the filler loading of 1 and 2 wt%. Prior to the spinning, the optimal dispersion time of CNC in N,N-dimethylacetamide was investigated based on the dynamic light scattering. Upon the bath-type sonication, the hydrodynamic diameter (Dh) of CNCs was reduced sharply from 1281 to 37.8 nm up to the dispersion time of 48 h, indicating the well-dispersed nature of the individual CNC molecules. After 48 h, the Dh of CNC plateaued in the range of 28.2-43.8 nm, and therefore, the optimal sonication time was set to 48 h. The tensile properties of dry-jet wet-spun PES fibers were significantly affected by the post-drawing temperature. Over the range of 80-120 °C, the 100 °C-drawing resulted in the highest tensile properties of the fibers. At a total draw ratio of 8, the control PES fibers exhibited the tensile strength of 230 MPa, modulus of 4.0 GPa, and toughness of 36.4 J/g, while PES/CNC1 fibers showed 240 MPa, 4.7 GPa, and 51.8 J/g, respectively. On the contrary, the incorporation of 2 wt% CNC led to diminished tensile properties due to the inhomogeneity of the spinning dope as confirmed by the rheological analysis.