This study aims to introduce inquiry-based play activities and to examine their effectiveness on young children's conceptual understanding of the shape of the Earth and the day-night cycle. The inquiry-based play activities for preschool children were designed to support children to learn the concepts of the shape of the Earth and the day-night cycle. The activities were designed based on the Preschool Learning Cycle (Play, Explore, and Discuss). The study sample consisted of 80 children, 33 of whom are girls and 47 are boys. The children's ages ranged from 54 to 70 months. To measure children's conceptual understanding, Earth-2 (EArth Representation Test for cHildren-2) developed by Straatemeier et al. (2008) was used. Science journals were used to promote children's reflective thinking and to evaluate their learning. The results revealed that the inquiry-based play activities positively affected young children's conceptual understanding of the shape of the Earth and the day-night cycle.