This study aims to examine the effect of "Formative Assessment (FA)" practices in "Assessment and Evaluation in Education" class on the academic achievement levels of prospective teachers. It uses a mixed research design. Quantitative data were collected by using double pretest-posttest design, which is one of the complete experimental design structures; while a semi-structured Interview Form was used to collect the qualitative data. The study group consists of a total of 220 prospective teachers who participated in a "Teacher Training Course" in Faculty of Education in Yildiz Technical University, Turkey during the 2017-2018 academic year. The data collection instruments included a 40-item multiple-choice achievement test (AT) chosen from a question bank in accordance with the course objectives and a semi-structured interview form. For the achievement test, reliability was achieved by the test-retest method (r=0.95), and validity was secured by the "analytical" method based on expert opinion. Following the nine-week FA practices using the complete experimental double pretest-posttest research design, it was found that these practices (what do I recall? and what have I learned?) resulted in a significant difference in favor of the experiment groups. The responses of prospective teachers to the semi-structured interview form developed to collect qualitative data for the study were categorized into common themes, which demonstrate that the quantitative data are confirmed by the qualitative data regarding the FA practices. This harmony between the quantitative and qualitative data showed that FA practices (independent variable) are strong enough to affect the achievements of prospective teachers (dependent variable).