Artificial graphite (AG) and natural graphite (NG) have different physical properties, which should have significant effects on their unique electrochemical characteristics: NG is known to have superior specific capacity and low cost, whereas AG shows superior cyclability and low-swelling characteristics. Herein, we present a straightforward strategy using triphenylphosphine to improve NG, enabling its physical properties to resemble those of AG and thereby improving its cyclability and swelling characteristics. The volume of a-few-nanometer-sized pores of NG is significantly reduced by triphenylphosphine treatment, which is similar to that of the AG. As a result, triphenylphosphine-treated NG (TPP) anode shows improved cyclability (83.6% after 300 cycles) compared to that of NG, confirming the correlation between nanopore volume and cyclability. Furthermore, the expansion of single-layer pouch cell prepared with TPP, as monitored by in situ thickness measurement, is suppressed to 9% after 55 cycles, which is superior to the pristine NG showing 14% expansion after the same number of cycles. Electrochemical impedance analyses of symmetric cells reveal that the changes of ionic and solid-electrolyte interface (SEI) resistance are accompanied by the reduction of nanopores, confirming that the formation of decreased SEI layer is a crucial factor for realizing less-swelling and highly durable anodes based on the cost-effective NG materials.