This work examines on the study of the phytochemical screening of the plant extracts recovered from the flowers of Crataegus oxyacantha. This herb belongs to the Rosaceae family. Preliminary phytochemical analysis of plants extracts showed that this plant has a wealth of flavonoids. Thus evaluation of the antimicrobial activity has only touched the flavonoids. The antimicrobial effect of flavonoids is evaluated on several microorganisms. It has been tested on eight bacterial strains: Staphylococcus aureus, Klebseilla pneumoniae, Bacillus subtilis, Escherichia coli and Pseudomonas aerugenosa. This extract has been studied on two yeasts namely Candida albicans and Saccharomyces cerievieae. It should be noted that these agents are characterized by a high frequency of contamination and pathogenicity. This study shows that the flavonoids extracts have antibacterial effects against Gram + and some Gram - except E. coli and P. aeruginosa. Thus an inhibitory effect on yeasts should be mentioned. This excerpt doesn't have any inhibitory power on Saccharomyces cerievisea.