Sodium metal holds promise as the ultimate anode for high-energy-density Na battery systems. Recent progress has been made in terms of rational design of nanostructured 3D current collectors for dendrite-free Na deposition with limited dimensional changes during cycling. However, critical information such as Na nucleation and growth behavior on these hosts remains elusive. Herein, by using amorphous carbon nanofibers (CNF) as a current collector, we present the first nanoscale-resolution observation of electrochemical Na plating/stripping dynamics via in situ electron microscopies. With the use of solid electrolyte, Na metal was found to grow and dissolve reversibly as nano/micro-particles at all the possible locations around indiviual CNFs and even throughout their network. Notably, inter-fiber Na ion transport was experimentally confirmed, which enables more homogeneous Na deposition deep into the network interior without interfacing the electrolyte; this would be crucial for dendrite-free Na plating, especially in all-solid-state Na batteries. In addition, through a delicately designed in-situ experiment, the CNF interior exhibited a superior Na capacity compared to its graphitized counterpart. Thus, owning to both exterior and interior Na storage of each fiber, CNFs could be a promising host material for building rechargeable composite Na metal anodes with ultrahigh capacity.