The purpose of this study is to explore preservice elementary mathematics teachers' conceptions regarding the use of history of mathematics as a teaching tool, especially Al-Khwarizmi's method of completing the square in the learning and teaching of quadratic equations. The case study of qualitative research methods was used. The data were collected through semi-structured interviews with 10 teacher candidates. The results of the study revealed the conceptions of the influences of the use of history of mathematics, specifically Al-Khwarizmi's method of completing the square, in the teaching of quadratic equations on preservice elementary mathematics teachers' current mathematical knowledge, as well as the conceptions of the influences of the use of this method as a teaching instrument on their future teaching professions. It has been seen that using Al-Khwarizmi's method in mathematics teaching has provided prospective teachers with better understanding of the second-order equations by presenting different techniques to integrate them into teaching quadratic equations.