The　self-assembly　of　nanoparticles　can　effectively　control　morphology　and　surface　exposure　during　materials　processing　and　has　promising　potential　applications　in　the　synthesis　and　exploration　of　different　types　of　materials.　As　the　process　often　occurs　in　liquid　solvents,　it　is　difficult　to　observe　and　study　the　dynamic　process　and　mechanism　of　the　self-assembly　of　nanoparticles　in　situ.　With　the　recent　development　of　environmental　transmission　electron　microscopy　(TEM),　researchers　have　been　able　to　observe　and　study　the　dynamic　processes　of　various　chemical　reactions　in　liquid　environments　in　real　time　on　the　nanometer-　and　atomic　scale.　In　this　paper,　we　used　advanced　in　situ　liquid　environmental　TEM　to　characterize　the　self-assembly　process　of　Co3O4　nanorods　in　water　and　study　the　mechanism　of　self-assembly.　A　self-assembly　phenomenon　of　the　Co3O4　nanorods　in　water　was　discovered　under　the　influence　of　electron　beam　irradiation　that　caused　a　change　in　the　dielectric　constant　and　increased　the　electrical　conductivity　in　the　irradiated　water.　The　movement　of　the　nanorods　was　initiated　and　energized　in　the　irradiated　water.　As　the　distance　between　the　nanorods　decreased,　the　drift　velocity　of　the　nanorods　and　the　interaction　force　between　them　increased.　By　analyzing　the　nanorods＇　movement　(including　the　distance　and　the　rotation　angle)　as　a　function　of　time,　the　variation　of　the　mean　square　displacement　with　respect　to　time　was　obtained.　Based　on　the　Stokes-Einstein　equation,　the　driving　force　was　estimated,　and　we　found　that　the　driving　force　increases　with　the　decreasing　distance　and　that　the　maximum　value　of　the　driving　force　is　approximately　12　pN.　By　characterizing　the　morphology　of　the　Co3O4　nanorods,　we　found　that　the　exposed　crystal　facets　of　the　nanorods　are　mostly　in　the　{100},　{110},　and　{111}　planes.　As　Co3O4　is　a　polar　metal　oxide,　these　crystalline　planes　tend　to　carry　certain　electric　charge　according　to　the　terminative　Co　or　O　atoms.　In　a　conductive　aqueous　environment,　it　is　because　of　these　easily　exposable　surfaces　with　different　surface　charges　that　the　Co3O4　nanorods　will　attract　each　other　under　the　driving　force　of　opposite　electric　charges.　Furthermore,　self-assembly　of　nanorods　with　a　complementary　morphology　can　be　driven　quickly.　The　polarity　of　the　residual　surface　charge　plays　a　decisive　role　in　the　effective　assembly　of　nanorods.　It　is　already　known　that　surface　charge　compensation　takes　place　on　polar　crystal　surfaces;　however,　this　work　provides　a　deeper　understanding　of　the　incomplete　nature　of　this　surface　charge　compensation.　The　results　presented　here　may　provide　important　experimental　data　and　theoretical　reference　for　artificially　regulating　the　metal　oxide　self-assembly　process.