Using　the　spark　plasma　sintering　technique,　the　dense　nanocrystalline　SmCo7　alloys　with　single-phase　structure　were　prepared.　Starting　with　the　prepared　nanocrystalline　single-phase　SmCo7　alloy　and　the　annealing　treatments,　the　changes　of　the　microstructure,　lattice　parameters　and　phase　stability　of　the　dopant-free　SmCo7　magnetic　alloy　were　investigated.　It　is　discovered　that　the　SmCo7　(1:7H)　phase　has　a　good　single-phase　stability　from　the　room　temperature　up　to　600°C.　At　higher　temperature,　as　a　concomitant　process　with　the　rapidly　coarsened　grain　structure,　the　SmCo7　phase　decomposes　into　Sm2Co17　(2:17R)　and　SmCo5　(1:5H)　phases,　and　the　1:5H　phase　exists　as　particulate　precipitates　distributing　uniformly　in　the　matrix.　In　addition,　it　is　found　that　both　the　lattice　parameter　a　and　cell　volume　of　1:7H　phase　exhibit　a　linear　decreasing　relationship　versus　the　annealing　temperature　in　the　range　of　400-700°C,　while　the　c/a　value　changes　a　little.