The　microstructures　of　ultrafine　WC-Co　cemented　carbides　were　characterized　and　analyzed　by　stereological　methods.　The　quantitative　relationships　between　the　mechanical　properties　and　microstructure　parameters,　such　as　the　mean　grain　size　d(WC),　mean　free　path　L-Co　and　contiguity　CWC-WC,　were　obtained.　The　calculated　results　of　mechanical　properties　agree　well　with　the　experimental　measurements.　With　the　equivalent　contiguity,　the　Vickers　hardness　of　ultrafine　cemented　carbides　has　linear　relationships　with　d(WC)(-1/2)　and　L-Co(-1/2)　and　the　fracture　toughness　K-IC　has　deterministic　relationships　with　d(WC)(-1/2)　and　L-Co(-1/2),　respectively.　With　certain　Co　content　and　mean　grain　size,　the　transverse　rupture　strength　decreases　with　the　increase　of　contiguity.　Especially　when　the　contiguity　is　above　0.5,　the　strength　decreases　obviously.　The　present　results　may　be　used　as　criteria　to　optimize　the　microstructure　and　to　improve　mechanical　properties　of　ultrafine　cemented　carbides.