Digital technology is an important factor for the development of modern education systems, especially the wireless communication technology and the sensor technology, which make the evolution of learning environments from web-based learning to mobile learning or ubiquitous learning. In this paper, a context-aware ubiquitous learning environment which utilizes mobile devices, sensors and wireless networks to conduct situated learning. Moreover, a series of learning activities of a butterfly ecology unit of the natural science course for K-4th students has been developed and conducted in the learning environment, in which students are guided to observe real-world objects with personalized supports from the system. Some preliminary experimental results have shown the effectiveness of the novel approach.