This study aimed to prove the antimicrobial effects of the natural substance Glycyrrhiza uralensis extract by applying it to Streptococcus mutans (S. mutans), for the prevention and treatment of dental caries. S. mutans (KCTC 3065/ATCC 25175) was incubated in Brain Heart Infusion (BHI) for 24 h. Glycyrrhiza uralensis was extracted with 80% ethanol and the diameter of the growth inhibition zone, the number of Colony-Forming Units (CFUs) and the viability were measured. After checking the antimicrobial effects of Glycyrrhiza uralensis extract on S. mutans, it was found that the diameters of the growth inhibition zone for the control (BHI), 2, 4, 6, 8, and 10 mg/mL were 0, 11, 13, 14, 15, and 18 mm, indicating that the growth was inhibited from 2 mg/mL. The number of CFUs was 3.2 × 1012 for 0 mg/mL, 3.2 × 104 for 2 mg/mL, 38.2 × 101 for 4 mg/mL, 5.3 for 6 mg/mL, 3.3 for 8 mg/mL, and 1 for 10 mg/mL, showing a decrease in the number of CFUs with the increase of the concentration of the extract. The viability was 0% from 2 mg/mL, showing a killing effect at all the concentrations. Based on the results of this study, Glycyrrhiza uralensis extract inhibited the growth of S. mutans from a 2 mg/mL concentration, and as the concentration of the extract increased, the number of CFUs in the Glycyrrhiza uralensis extract tended to decrease. Moreover, growth inhibition of the Glycyrrhiza uralensis extract was observed from a 2 mg/mL concentration. Based on the study results, it can be concluded that Glycyrrhiza uralensis extract can prevent dental caries by inhibiting the growth of S. mutans.