Ubiquitous computing and mobile technologies provide a new perspective for designing innovative outdoor learning experiences. The purpose of this study is to propose a formative assessment design for integrating PDAs into ecology observations. Three learning activities were conducted in this study. An action research approach was applied to continually revise the worksheet designs. Twenty-seven 5th and 6th graders' observation skills using PDAs, and their extended inquiry performance using e-diaries were assessed. An e-library, and online scoring and feedback systems were developed accordingly. The results suggest that the embedded formative assessment designs were effective for guiding and supporting the students' learning progress. The activity worksheets successfully helped the students focus their outdoor learning attention on the target tasks. The e-library provided reliable resources to clarify their observed descriptions, while the automatic scoring and feedback systems were especially helpful in sustaining the students' persistent effort. Most students demonstrated substantial improvements in their observation skills, and extended their inquiry abilities. The well designed online formative assessment embedded in the activity worksheet is very promising for PDA integrated learning. (Contains 2 tables and 9 figures.)