The　cooling　curves　of　10　different　kinds　of　quenching　oils　and　the　hardness　of　samples　quenched　in　the　oils　have　been　measured.　It　was　shown　that　the　hardening　power　calculated　from　Segerberg　formula　gave　better　agreement　with　test　results　than　the　transition　temperature　from　vapor　stage　to　boiling　stage　and　the　maximum　cooling　speed.　The　hardening　power　also　showed　a　better　agreement　with　the　measured　value　for　carbon　steel　than　for　alloy　steel.　Based　on　the　idea　＂＇Keeping　rapid　cooling　within　a　specific　temperature　range　that　varies　with　different　steels　is　a　key　for　steel　hardening＂,　revised　formulae　for　carbon　and　alloy　steels　were　put　forward　respectively.　Test　results　suggest　that　the　revised　formulae　are　better　than　the　original　ones.