The purpose of this study is to investigate the effects of a ubiquitous problem-based learning system (UPBLS) on students' question-raising performance in field inquiry activities. An experiment was conducted on an elementary school natural science course. A total of 43 sixth and fifth grade elementary students divided into experienced and novice groups participated in the field observation activities with on-line discussion over a period of seven months to evaluate the changes in their questioning ability. Moreover, a rubric for evaluating the students' questioning ability was developed and validated. Supported by UPBLS, the students collected the required wetland ecology data in three wetland field observation activities. The experimental results show that the students' questioning abilities significantly improved during the learning process. Moreover, it was found that both the experienced and the novice students had similar progress trends, implying that the proposed approach is helpful for improving the questioning abilities of students with different levels of prior knowledge.