The research that studies the improvement of elementary school students' science knowledge and the development of elementary students' abilities to use the scientific and engineering practices is restricted. The purpose of this research is to study the effects of an instructional intervention on the knowledge of the students of elementary school about electromagnets and their abilities to design science investigations. Instructional material about electromagnets was developed, based on an inquiry-based approach using the scientific and engineering practices, which was applied to 76 students of elementary school (12 years old). To evaluate the students' knowledge of electromagnets as well as their abilities to design science investigations, a questionnaire was developed which was completed by the students both before and after the instructional intervention. The data of the research were the answers of the students to the questionnaires. The data analysis showed that the students' knowledge about electromagnets as well as their abilities to design science investigations was improved, through the instructional intervention for electromagnets.