This research aims to developing STEM-AR-based e-book teaching materials to
improve students' scientific reasoning in ecology and biodiversity materials. The
development of this research is important as a parameter of 21st-century abilities, which
require students to have 4C abilities to face the education era of 4.0. This research method
uses Research and Development (R&D). The R&D design chosen was Thiagarajan design
consisting of four stages (Define, Design, Develop, and Disseminate) (Thiagarajan, 1974).
Class VII students (N = 42) were involved as research subjects who were taken by the
purposive sampling technique. Data collection was carried out through scientific reasoning
tests, student response questionnaires, and observation sheets. Data processing techniques
used paired sample T-tests and then performed the N-gain test. The results of the analysis
show that after using the STEM-AR-based e-book, there is an increase in scientific reasoning
ability, with an N-Gain value of 63% in the medium category. Testing the effectiveness of
teaching materials using the SPSS paired sample t-test showed a result of 0.001, which means
that STEM-AR-based teaching materials affect students' scientific reasoning. The descriptive
average of scientific reasoning has increased by 40%. In addition, students gave positive
responses to teaching materials, which made it easier to understand the material.