Oleanolic　acid　is　a　naturally　occuring　triterpenoid,　widely　used　as　an　OTC　drug　for　acute　and　chronic　hepatitis.　Its　solubility　in　ethanol-water　was　measured　at　293.15~338.15　K,　pH　7~10　and　ethanol　concentration　50%~90%.　The　solubility　increases　sharply　with　pH　value　from　8　to　10,　rises　with　increasing　ethanol　concentration　only　during　higher　pH　range　(9~10)　and　increases　slowly　with　the　increase　of　temperature　only　during　pH　9~10　and　ethanol　concentration　of　80%~90%.　pH,　ethanol　concentration　and　temperature　show　less　and　less　effects　on　the　solubility　and　hence　the　reaction　crystallization　by　adjusting　acidity　is　suitable　for　oleanolic　acid　purification.　The　famous　λ-h　equation　were　modified　with　two　empirical　functions　to　include　the　effects　of　pH　value　and　ethanol　concentration　on　the　solubility,　by　which　80　groups　of　experimental　solubility　data　were　well　correlated　with　a　relative　coefficient　of　0.986　and　an　average　error　of　6.3%.　The　reaction　crystallization　metastable　zone　of　oleanolic　acid　in　ethanol-water　under　room　temperature　was　determined　by　using　laser　monitoring　technique.　The　metastable　zone　width　increases　linearly　with　increasing　saturation　concentration　of　oleanolic　acid　with　a　correlation　coefficient　up　to　0.998.