When　extracting　stiff　clays　at　a　certain　depth,　negative　water　pressure/suction　builds　up　inside　the　samples.　If　the　generated　suction　is　high　enough　to　desaturate　the　largest　pores,　the　soil　sample　will　be　desaturated.　From　a　practical　point　of　view,　it　is　essential　to　investigate　the　effect　of　degree　of　saturation　S-r　on　the　soil　mechanical　behaviour.　In　this　study,　nine　boreholes　of　stiff　clays　were　taken　from　different　depths.　The　unconfined　compressive　strength　(UCS)　was　determined　for　soils　at　different　depths.　A　first　analysis　showed　that　there　is　no　well　established　relationship　between　UCS　and　S-r.　To　understand　this　observation,　the　water　retention　property　and　compression　behaviour　were　further　investigated　to　determine　the　main　air　entry　value　(AEV)　and　the　over　consolidation　ratio　(OCR).　Soil　microstructures　were　observed　through　mercury　intrusion　porosimetry　(MIP).　Based　on　the　results,　the　sampling-induced　suction,　the　void　ratio　of　air　at　main　air　entry　value　e(M.AEV),　the　void　ratio　of　water　at　main　air　entry　value　e(m-AEV),　the　water　ratio　e(w),　and　the　air　ratio　e(a)　were　estimated.　The　e(M.AEV)　(or　e(a))　and　the　e(m-AEV)　(or　e(w))　were　selected　to　analyze　the　results　of　UCS.　It　appears　that　UCS　is　controlled　by　e(m-AEV)　or　e(w)　when　S-r　is　higher　than　the　one　corresponding　to　the　main　air　entry　value.　By　contrast,　in　case　of　lower　S-r,　UCS　is　rather　controlled　by　e(M.AEV)　or　e(a).　This　suggests　that　S-r　has　a　significant　effect　on　UCS　only　when　the　soil　suction　is　beyond　the　main　air　entry　value,　UCS　being　rather　controlled　by　soil　dry　density　at　lower　suctions.　This　finding　provides　useful　information　for　analyzing　the　mechanical　behavior　of　natural　soils　in　geotechnical　practice.