This study was undertaken to explore the effects of informal science learning outside the classroom on preschool students' achievement in the Early Science learning topic (plant-related topics that presented concepts about tree leaves, height and roots) using an inquiry method. A sample of 64 preschool students was selected using purposive sampling in a quasi-experimental research method with non-equivalent control and treatment groups. There were 32 preschool students in the treatment group and 32 preschool students in the control group. The treatment group was taught the Early Science topic outside the classroom using inquiry while the control group was taught the same topic by conventional methods in the classroom. The findings indicated that there were significant differences between the two groups of preschool students in the mean score of the post-test on leaves' shape, plant height and root positions. This finding showed that Early Science learning through informal science learning outside the classroom using an inquiry method can improve students' achievement in the Early Science topic related to plants (concepts about tree leaves, height and roots). This study has implications for teachers using interactive teaching methods in order to engage students more actively in their learning process. In addition, this study has implications for school administrators in providing a learning environment that will enhance the preschool students' achievement in Early Science learning.