© 2020 Elsevier Ltd Na metal is an attractive anode material for rechargeable Na ion batteries, however, the dendritic growth of Na can cause serious safety issues. Along with modifications of solid-electrolyte interphase (SEI), engineering the electrode has been reported to be effective in suppressing Na dendritic growth, likely by reducing localized current density accumulation. However, fundamental understanding of Na growth at the nanoscale is still limited. Here, we report an in-situ study of Na electrodeposition in electrochemical liquid cells with the electrodes in different surface roughness, e.g., flat or sharp curvature. Real time observation using transmission electron microscopy (TEM) reveals the Na electrodeposition with remarkable details. Relatively large Na grains (in the micrometer scale) are achieved on the flat electrode surface. The local SEI thickness variations impact the growth rate, thus the morphology of individual grains. In contrast, small Na grains (in tens of nanometers) grow explosively on the electrode at the point with sharp curvature. The newly formed Na grains preferentially deposit at the base of existing grains close to the electrode. Further studies using continuum-based computational modeling suggest that the growth mode of an alkali metal (e.g. Na) is strongly influenced by the transport properties of SEI. Our direct observation of Na deposition in combination with the theoretical modeling provides insights for comprehensive understanding of electrode roughness and SEI effects on Na electrochemical deposition.