Science festivals have occupied a very important axle in informal science education that enables students to experience the amazement of scientific experiments to think over scientific principals beyond the formal education in the classrooms. Among the concerned person, the most benefit-taken group may be the assistant who help the participants experience the activities in the festival. In order to find out the ways to make the student assistant`s participation into a meaningful education experience, we analyzed the types of the activities in the science festival as well as the characteristics of the interaction between the student-assistants and the participating students are studied. The research findings are as follows: First, most activities in the science festival had related to the scientific concepts or principals; however, the understanding of the concepts and principals didn`t highly affect the procedure of the activities. In many cases the students operated and made results without checking the related concepts or principals. Second, the student-assistants showed the consistency of operation in guiding their activities. They were explaining mainly the process of the experiments without giving a chance to think of related concepts or principles. We suggest that teacher should consider the student-assistants` learning in the festival as well as that of the participants.