The study included preparation the aqueous extract of thuja orientalis fruit in different concentrations ranged between (6.25 to 150) mg / ml , and detect active groups in it. The extract appeared containing (Flavonod , phenols, terpenes, saponins , glycoside and tannins). The inhibition activity of the extract against microorganisms isolate tested the Gram negative bacteria Pseudomonas aeruginosa and Escherichia coli and Gram positive Bacillus spp. and Staphylococcus aureus. The minimum inhibitor concentration of extract against tested bacteria was 12,5 mg/ml. Pseudomonas aeruginosa was studied because it causes the water pollution problems and highly resistance to many disinfectants and antibiotics. The antibacterial activity of the aqueous extract in different concentrations was tested against Pseudomonas aeruginosa bacteria (isolated from the water chairs dentist apparatus tank). The results showed that the best bactericidal concentration was 25 mg/ml with high significant (p≤0.001) and the best inhibitor concentration was 12,5 mg/ml. The extract kept its antibacterial activity for 24 hours.