Oxygen　injection　into　environmental　scanning　electron　microscopy　(ESEM)　effectively　reduces　or　eliminates　charging　effect　on　insulating　sample　surfaces　under　electron　beam　irradiation.　Secondary　electron　images　clearly　indicate　that　oxygen　works　much　better　than　water　vapor　in　charge　compensation　on　many　insulating　surfaces,　including　metal　oxides,　Al2O3,　metal　hydroxides,　Al(OH)　and　biological　samples　in　a　pressure　ranging　from　130　to　600　Pa　in　ESEM.　Current　absorption　Ia　was　measured　in-situ　to　understand　the　mechanism　of　charge　compensation　in　O2　environment.　Under　electron　beam　irradiation,　high　density　of　surface　electron　traps　form　because　electron　stimulated　desorption　results　in　oxygen　depletion　and　energy　band　distortion　on　the　surface.　However,　in　oxygen　environment,　these　oxygen　vacancies　can　be　easily　filled　by　O　ions.　As　a　result,　the　charging　effect　can　be　effectively　eliminated.