The present disclosure relates to a battery module, including a cell assembly including one or more battery cells and a busbar assembly electrically connecting the one or more battery cells, a module case including an opening part at least partially opened on one surface, and forming an accommodating space for accommodating the cell assembly therein through the opening part, a module cover coupled to the module case to close the opening part, and a filling part which is provided between an outside of the busbar assembly and the module case and includes an electrically insulating material, and an assembly method thereof.