Introduction:The concept of microbial ecological changes and probiotic approaches is a promising concept for preventing dental disease and decreasing of oral cavity pathogens. Aim: to evaluate the effect of probiotic strain Lactobacillus on the count of Streptococcus mutans and Lactobacillus in saliva and the plaque formation rate. Materials and methods: This randomized clinical trial included 30 normal apparently healthy and cooperative children aged 6 to 11 years and selected from the