This study aimed to design mediated prompts as dynamic assessment (DA) intervention to enhance young students' English-speaking abilities in storytelling tasks and to explore the effects of the designed mediated prompts as DA intervention on English-speaking abilities of young students. The research was conducted in two phases. The first phase illustrated the design process of mediated prompts, and the adjustments made for pre-scripted mediated prompts. Ten students from K3 level were selected as participants for this phase. In the second phase, twenty K3 students were selected to assess the impact of DA intervention. This research was conducted in an international school in Thailand for a duration of 8 weeks. The results revealed the value of DA intervention in finding students' potential weaknesses and confirmed the positive effect of utilizing mediated prompts on students' English-speaking abilities. The intervention promotes an English learning environment in the classroom that is joyful, stimulating, and stress-free. This study can benefit educators by providing insights into designing and applying mediated prompts during DA intervention in storytelling tasks for young students, ultimately improving their English-speaking abilities.