A　series　of　rare　earth　bulks　with　the　ultrafine　nanocrystalline　structure　were　prepared　by　applying　an　＂oxygen-free＂　(an　environmental　oxygen　concentration　less　than　0.5　ppm)　in-situ　synthesis　system,　where　the　inert-gas　condensation　was　combined　with　the　spark　plasma　sintering　technology　into　an　entirely　closed　system.　The　thermal　and　mechanical　properties　of　the　prepared　ultrafine　nanocrystalline　bulks　were　characterized　and　compared　with　those　of　the　raw　polycrystalline　bulks.　It　was　found　that　the　specific　heat　capacity,　the　microhardness,　and　the　elastic　modulus　of　the　ultrafine　nanocrystalline　rare　earth　bulks　were　significantly　increased　from　the　conventional　polycrystalline　bulks.