Objective: recently, researchers have increasingly turned to study the relation between stuttering and utterance length. This study investigates the effect of utterance length on the amount of speech dysfluency in stuttering Persian-speaking children and adults in conversational speech. The obtained results can pave the way to reach a better understanding of stuttering of child and adults, as well as finding more appropriate treatments. Materials & Methods: in this descriptive- analysis study, the participants were 15 stuttering Persian- speaker adults, upper from 15 years old, and 15 stuttering Persian- speaker children in the age range of 4-6. In this study, first 30 minutes sample of adults and child's spontaneous speech was provided and then utterances of each person studied for the amount of dysfluency and utterance length. The obtained information intered to computer via spss software and analyzed using paired T test. Results: In both groups of stuttering children and adults, with increase of utterance length, there was a significant increase in the amount of dysfluency. Conclusion: The results of this study showed that by increase of utterance length at the spontaneous speech level, stuttering children and adults had more dysfluency amount. Also, by increase of utterance length, dysfluency amount of stuttering children and adults increased samely.