The invention relates to the effective parts of rosemary, its preparation method and use, wherein the effective parts of the Rosmarinus officinalisl are diterpenoids phenols extracted from medicinal material of Rosmarinus officinalisl, the content of the phenols is over 45%, and the sum of the content of the carnosol and carnosic acid as the principal ingredients is over 13%. The preparing process comprises curing the Rosmarinus officinalisl medicinal material and cutting it into chunks, charging raffinate for extraction, cooling down the merged extract and filtrating, decolorizing with activated charcoal, concentrating the filtrate and charging in water, filtering, drying and disintegrating the filtering deposit. The effective parts of the Rosmarinus officinalisl can be used for preparing medicament for preventing and treating cardiovascular diseases, epigastric pain, uterocarcinoma, cervical carcinoma, intestinal cancer and breast cancer.