We conducted a 30-minute lecture about forests in an outdoor experience program for elementary school children, and examined how the content of the lecture affected children's knowledge and behavior. The subjects were 29 elementary school children who participated in the “Uminoko Outdoor Program” organized by the Rikuchu-Kaigan Youth House in Iwate Prefecture. The main part of this program is a marine experience, but it also includes the stream experience activity. We gave a lecture about the topic of trees and forests to children the day before the stream experience. Then, we grasped how the contents of this lecture affected the behavior of the children during the stream experience through behavior observation surveys and interview surveys. We also conducted a questionnaire survey to understand the impact of the lecture on children's knowledge after the program. As a result, we could not confirm the behavior of the children affected by the lecture during the stream experience. However, in questionnaire survey of parents about 50 days later, we confirmed that about half of the children were telling their parents the content of the lecture.