The present in vitro study determined the antibacterial effect of Coconut (Cocus nucifera) oil on Streptococcus mutans ATCC 25175. Cross-sectional and experimental study. Streptococcus mutans strain ATCC 25175 was seeded on Müeller-Hinton Blood Agar. The effect was determined by inhibition halo and the minimum inhibitory concentration, using procaine penicillin G as a positive control and standard suspension of Streptococcus mutans as a negative control. Twelve replicates per Coconut oil concentration (25 %, 50 % and 75 %) were performed. Coconut oil concentration at 25 % generated an inhibitory mean of 17 mm and 2.23x10 2 CFU, at 50 % a mean of 21.75 mm and 0.17 x 10 2 CFU, at 75 % a mean of 22 mm and 0 CFU, procaine penicillin G a mean of 14.25 mm and 0 CFU, the negative control gave a mean of 2.8 x 10 5 CFU. The Kruskal-Wallis non-parametric test detected a highly significant statistical difference of the three concentrations of Coconut oil (p < 0.01). The Mann-Whitney test with Bonferroni adjustment determined that the 50 % and 75 % concentration had similar inhibitory action and that both the 75 % concentration and procaine penicillin G gave a mean of 0 CFU. All concentrations of Coconut oil showed inhibitory action on Streptococcus mutans ATCC 25175. 75 % concentration showed the highest inhibitory mean and was the minimum inhibitory concentration that completely eliminated CFU.