In the increasing pervasiveness of today's digital society, mobile devices are changing the face of education. Students can interact with mobile devices in context-aware environment. This paper presents a mobile application based on expert system (Plant-E) for students to acquire knowledge about plant classification by answering decision-making questions. In order to study effectiveness of Plant-E and cognitive load of students who experience different kinds of interaction in learning process, another mobile application (Plant-G) only providing information pages of plants was developed. A quasi-experiment was conducted with three classes of 137 seventh graders. The three classes, Class A with 46 students using Plant-E in campus, Class B with 44 students using Plant-G in campus, and Class C with 47 students using Plant-G in a traditional classroom, constitute three groups with different interaction complexity. The research conducted pretest, posttest, and delayed test to evaluate learning performance of students in three classes and used a questionnaire to investigate their perceptions and attitudes towards proposed system. Results show that interaction complexity has an impact on students' learning performance and mental effort in mobile learning; the higher the interaction complexity is, the higher mental effort and the better learning performance in mobile learning will be.