Academic researches about growth-mindset interventions show that growth-mindset strategies can be effective on academic achievement. However, it is not clear yet how this effect occurs and what is the background of this. The aim of this research is to examine the effect of the growth-mindset intervention including growth-mindset strategies on the academic achievement and mindset scores of 8th grade students who are continuing secondary education in Turkey by using mixed method. The sample of the research, which was designed in accordance with the embedded mixed design, consists of 42 8th grade students (Experimental group: 21, control group: 21). Within the scope of the research, an extracurricular 8-week training was applied. Mindset scale and academic achievement scale were applied before and after the intervention. At the same time, qualitative data were collected through semi-structured interviews and observations before, during and after the intervention. The findings obtained at the end of the research show that growth-mindset strategies make a significant change on students' academic success and mindset scores.