The　crystal　structures,　electronic　structures,　cohesive　energies,　and　the　Bader　charges　of　fluorite　ZrO2　and　Ru-doped　ZrO2　composite　oxide　were　investigated　by　using　projector　augmented　wave　method　and　the　generalized　gradient　approximation　based　on　density　functional　theory.　The　results　show　that　the　calculated　equilibrium　lattice　parameters　of　cubic　ZrO2　and　RuO2　are　in　good　agreement　with　the　available　literatures.　By　adding　Ru　into　fluorite　ZrO2,　the　cell　shape　changes　slightly　and　cell　volume　is　reduced,　the　system　changes　from　direct-band-gap　semiconductor　to　indirect-band-gap　semiconductor,　and　the　forbidden　gap　width　is　reduced　from　3.6　eV　to　1.03　eV.　Comparing　the　changes　of　cohesive　energy,　density　of　states　and　Bader　charges　of　ZrO2　before　and　after　doping　shows　that　the　ionic　character　is　enhanced　and　the　covalent　character　is　weakened　by　adding　Ru　into　fluorite　ZrO2.