Objective: To extract effective constituent from Magnolia officinalis and investigate its inhibitory effects on growth of main cariogenic bacteria. Methods: Two main fractions (MO1, MO2) of M. officinalis were extracted, isolated, and identified by means of chemical fractionation and analysis. Their antimicrobial activity was determined by the disk agar diffusion method and the zones of growth inhibition were measured. Through micro-liquid dilution the minimal inhibitory concentrations (MIC) and minimal bacterial concentrations (MBC) of MO2 against various types of Streptococcus mutans were examined. Results: Effective ingredient (MO2) was isolated from M. officinalis and was identified as diphenyl compounds composed of magnolol and honokiol. MO2 produced significant inhibitory effects on all the microorganisms tested (P < 0.05). The MIC and MBC of MO2 were not statistically different from authentic magnolol (P > 0.05) although second to triclosan (P < 0.05). Conclusion: MO2 was an effective constituent of M. officinalis and possessed powerful inhibitory effects on the growth of various serotypes of S. mutans.