Sixty six Streptococcus mutans isolates have been tested with cephalexin and Salvadora presica L extracts with or without addition of glucose, or xylitol to determine minimal inhibitory concentration (MICs), susceptibility of S. mutans isolates and time-killing curve. Result: All the strains were sensitive to cephalexin (12.8. ug/ml) , S. presica aqueous extract (16.0 mg/ml), S. presica methanol extract (4.0 mg/ml), S. presica ethanol extract (12.0 mg/ml) and S. presica hexane extract (8.0 mg/ml). The addition of xylitol increased susceptibility of S. mutans isolates to the MIC values of cephalexin and S. presica extracts, whereas the addition of glucose decreased susceptibility of S. mutans to the MIC values of cephalexin and S. presica extracts. Time-killing curve of cephalexin combined with xylitol exhibit zero CFU/ml after 16 hrs. and xylitol combined with S. presica hexane extract exhibit zero CFU/ml after 18 hrs with highly significant values. Conclusion: The mix of xylitol with S. pre sica extracts or cephalexin may lead to inhibit microbial causative agents of dental decay, while addition of glucose has opposite effect.