The effect of mechanical grinding on the electrochemical properties of P100 and P75 fibers and natural graphite UF4 was studied. The electrochemical experiments were conducted in the NaClO4-ethylene carbonate (EC) electrolyte. Graphite did not allow significant sodium intercalation intrinsically but P100 and P75 fibers, which display lower graphitization level lead to richer compounds. A moderate mechanical grinding of P75 fibers resulted in the appearance of an extra reversible capacity corresponding to sodium insertion in the amorphous and porous parts of the material. Such a treatment allowed to change the composition from Na0.22C8 to Na0.31C8. With graphite, efficient grinding allowed this material to accommodate large amounts of sodium. Thus, the composition Na0.06C8 obtained with as-received graphite was changed to Na0.67C8 after grinding.