108　sub-areas　were　selected　from　three　major　sample　areas:　Yanchuan,　Suide,　and　Fu　County　which　are　different　in　physiognomy　and　locate　in　the　Loess　Hill　and　Gully　Area.　The　slope　matrix　derived　by　DEM　with　the　resolution　at　5　m　established　according　to　the　map　scaled　at　1:10000　was　taken　as　the　standard　value.　The　accuracy　of　the　slope　matrix　with　resolutions　from　15　m　to　75　m　based　on　single　grids　was　analyzed.　The　slope　accuracy　evaluating　indexes　based　on　grids　were　proposed　as　follows:　the　graded　slope　accuracy　a　and　slope　error　b.　The　function　of　the　above　error　indexes　and　the　horizontal　resolution　was　obtained　in　20　randomly　selected　experiment　areas.　According　to　the　function,　the　range　of　the　needed　resolution　was　calculated　when　the　errors　were　known.　The　function　has　been　tested　in　16　randomly　selected　areas.　The　results　show　that　indexes　a　and　b　can　be　used　to　evaluate　the　accuracy　of　the　slope　derived　by　DEM　based　on　single　grids.