Abstract: The content of free proline,peroxidase(POD)and the relative permeability of plasma membrane(RPM)in seedlings of Ginkgo biloba were studied under different levels of osmotic stress by PEG(6000,10%～30%).The results showed that the POD activity increased in first two days(except 10%),then decreased proline content increased from the second day to the sixth day especially under 20%～30% it increased sharply,while RPM increased greatly from the forth day.In the three indications,the peak of POD activity appeared most early,then proline content and RPM last.There was significant correlation between proline content and RPM( r =0.97)and a general correlation between proline content and MDA.As a result,three categories were classified in the four levels of osmotic stress.Ginkgo seedlings was able to endure the 20%PEG(6 000)level of water stress and would be harmed above that of level.