The aim of this research is to examine the effect of authentic investigation activities on the reasoning process of ratio concept which has been regarded as the milestone of mathematics and science teaching. In line with this purpose, the research has been carried out with eight 7th grade students attending a public school located in Central Anatolia Region in Turkey. The success levels of these students in mathematics are in a heterogeneous structure ranging from higher and moderate to lower levels. The data of this qualitative study has been collected through authentic investigation activities, researcher logs, and interviews and has been analysed via descriptive techniques. At the end of the study, it has been concluded that the teaching environment containing authentic investigation activities has a positive effect on the reasoning process of students on ratio concept.