In this research, we focused on testing the physical and mechanical properties of the devel-oped polyacrylonitrile (PAN) composite nanofibers with succinite (Baltic amber) and SiO2particlesusing standard methods of nanofiber testing (physical and mechanical properties). Polyacrylonitrilecomposite nanofibers (based on the electrospinning method) were coated on an aluminum substratefor structural investigation. SEM was used to determine the average fiber diameter and standarddeviation. The mechanical properties of the fibers were determined using a universal testing ma-chine (NANO, MTS). We observed that constant or decreased levels of crystallinity in the ultrafinecomposite nanofibers led to the preservation of high levels of strain at failure and that the strengthof nanofibers increased substantially as their diameter reduced. Improvements in PAN compositenanofibers with succinite and SiO2nanopowder are feasible with continuous decreases in diameter.The drastically decreased strain at failure demonstrated a substantial reduction in viscosity (tough-ness) of the annealed nanofibers. Large stresses at failure in the as-spun nanofibers were a result oftheir low crystallinity. As a result, decreasing the diameter of PAN nanofibers from approximately2 micrometers to 139 nanometers (the smallest nanofiber tested) resulted in instantaneous increasesin the elastic modulus from 1 to 26 GPa, true strength from 100 to 1750 MPa, and toughness from 20to 604 MPa.