Dental caries is the most important global problem of oral disease. The local demineralization of tooth surface from an acid action is an initial step of the disease. The acids are produced when sugar from food has an interaction with bacteria in the dental plaque, which usually accumulates on the susceptible tooth surfaces. As more acidic condition, the aciduric and acidogenic bacteria can survive. Streptococcus mutans is a major contributor of tooth decay. Many strategies are recommended to protect the susceptible teeth from cariogenic bacteria. A probiotic application is one of techniques providing the health-beneficial microorganism to inhibit the cariogenic bacteria. Prebiotics are oligosaccharides which can promote the growth of probiotics in human bowel. The objective of this study was to evaluate the efficiency of the prebiotic (Galacto-oligosaccharides, (GOS)) to enhance the probiotic (Lactobacillus acidophilus) for inhibition of S. mutans and L. acidophilus were co-cultured with ratio of 1:20 in the de Man, Rogosa and Sharpe (MRS) media supplemented with different concentrations of GOS; 1, 2, 3 and 4 % (v/v). The efficiency of synbiotic against S. mutans was determined from their growth rate. The growth rate of S. mutans and L. acidophilus were similar (0.4848 and 0.4861 hr-1, respectively) in the MRS agar without GOS. The growth rate of S. mutans insignificantly decreased when grew in 3 and 4 % of GOS (0.1719 and 0.3258 hr-1 respectively) compared with control group (p > 0.05), while the growth rate of L. acidophilus was constant (0.3443 and 0.3459 hr-1 respectively). The GOS was not an efficient prebiotic to enhance the function of L. acidophilus to inhibit growth of S. mutans.