Oxygen-carrying substances based on poly-placental hemoglobin were put into freeze-drying. Sucrose was chosen to inhibit the methmoglobin(MetHb) formation. MetHb content, ultravioletes(UV) spectrum, Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), High performance liquid chromatography (HPLC) were monitored during freeze-drying. As a result, when the mass ratio of sucrose to protein was above 0.5, MetHb formation was under control;the characteristic absorption of UV spectrum, SDS-PAGE and HPLC showed no visible change. Freeze-dried solids were kept under room temperature and refrigerator for 3 months. As a result, MetHb formation depended upon storage temperature and the mass ratio of sucrose to protein. For group C in which the mass ratio of sucrose to protein was 1.0; there is no marked change in MetHb content, UV spectrum, SDS-PAGE and HPLC after 3 months.