Aims: This study aimed to evaluate and compare the antibacterial activity (AA) of Origanum dubium and Cinnamomum cassia essential oils dissolved in different ratios of three different solvents against Streptococcus mutans. Methodology and results: The essential oils were obtained from O. dubium leaves and C. cassia barks using hydrodistillation method and gas chromatography/mass spectrometry (GC/MS) analyses were performed to identify the compositions. The obtained essential oils were diluted in dimethylsulphoxide (DMSO), Tween 20 and ethanol with concentrations of 10, 20 and 60% respectively. Each oil mixture was further diluted with distilled water to provide different ratios. Then, the AA of the dilutions were evaluated. Comparison of AA of essential oils showed that C. cassia had higher AA than O. dubium for each dilution ratio for Tween and ethanol. Conclusion, significance and impact of this study: The results obtained in our study lead us to affirm that the AA of both oils are dependent on dilution ratios of the solvents and the AA of C. cassia is higher than that of O. dubium except for the 1:8 and 1:16 dilution ratios of DMSO. Increasing ratios of solvents used to dilute the O. dubium and C. cassia essential oils resulted in a decrease in the antimicrobial activity against S. mutans.