Objective: The purpose of this clinical study was to evaluate the effectiveness of Lidcombe Program in reduction of preschool children’s stuttering severity. Method: This study was a single group semi-experimental clinical trial with repeated measurement, in which 10 preschool stuttered children were selected and were affected by Lidcombe Program within a period of 12 individual sessions. Children’s stuttering severity was measured according to the percentage of syllables stuttered (%SS) at three time points. Obtained data were analyzed according to ANOVA with repeated management. Results: The results showed that Lidcombe Program could reduce significantly preschool children’s stuttering severity (p<0/01). Conclusion: This study confirmed the effectiveness of Lidcombe Program in reduction of preschool children’s stuttering severity.