The purpose of this study is to investigate students' thinking of direct, inverse and nonproportional problems. Thirty two seventh grade students from three different government schools participated in this study. To collect the data, the participants were asked to solve 9 open-ended problems, including 3 direct, 3 inverse and 3 non-proportional problems. Dual-process theory was used to analyze the data. The results showed that in direct proportional problems students' answers were almost equally intuitive and analytical: in inverse and nonproportional problems their thinking was more intuitive, and with non-proportional problems, most intuitive and analytical answers were incorrect. In addition, the results showed that, in general, most students who provided incorrect answers to inverse proportional problems and especially non-proportional problems, had difficulty in distinguishing direct and inverse proportional problems and non-proportional problems because of using incorrect proportional strategies.