Abstract: The antibacterial activity of different ethanol extracts from Glycyrrhiza uralensis Fisch (G-EA) and Magnolia officinalis (M-EA) to Streptococcus mutans were tested and compared. The effect on the bacteria activity of lactic dehydrogenase (LDH) and the restrain of biofilm of the two ethanol extract compounds were explored as well. The antibacterial abilities of the two ethanol extracts and their complex were investigated through testing the inhibition zone, minimal inhibit concentration (MIC) and fractional inhibitory concentration (FIC). The results showed that FIC between G-EA and M-EA for Streptococcus mutans was 0.375 and possessed a synergistic effect. After the test of MIC values of different mass ratios of G-EA to M-EA, the bacteriostatic effect was the best for the complex against Streptococcus mutans when the mass ratio of G-EA to M-EA was 1∶2 (G-M_1∶2), and MIC of the complex reached 0.32 g/L. According to the growth curve test of Streptococcus mutans, the main bacteriostatic phase of G-M_1∶2 was its logarithmic growth phase and stationary phase, and the bacteriostatic effect of G-M_1∶2 to Streptococcus mutans was superior to the tea polyphenol at the same concentration. The effects of G-M_1∶2 on lactic dehydrogenase (LDH) activity and biofilm formation in Streptococcus mutans were investigated. It demonstrated that G-M_1∶2 could effectively inhibit LDH activity and biofilm formation, and the inhibition ability was enhanced with the increase in concentration of G-M_1∶2. The test also indicated the minimum biofilm inhibition concentration 50% (MBIC₅₀) of G-M_1∶2 was 0.08 g/L.