The observation of trees with identification is a challenging activity for teachers who lack knowledge about trees. Effects of usual activities on learners' skills in observation are not still understood specifically. In this study, a simple activity of observation of schoolyards trees without identification was developed for such teachers. The activity was performed for upper secondary school students by using an original worksheet, and the effects of the activity on the learner's viewpoints on trees were specifically evaluated. Most of students focused on leaves and branches without clear viewpoints of observation before the activity. After the activity, however, they focused not only on leaves and branches but also on other organs with various viewpoints of observation. They also learned the names of the schoolyards trees and transformed their images of the trees into clear pictures. This activity is evaluated to be effective for learners in improving their skills in observation, and it can be introduced into school lessons easily.