This correlational study is to show the role of deductive and inductive reasoningabilities as aspects of cognitive development on expository text comprehensionamong high school students who already have a capacity to think in conceptualand hyphotetical ways and to examine how they correlate and influence the textcomprehension process. This study has broaden our knowledge of languageand thinking, as it is different from other previous studies that focused oncomprehension process among elementary students, narrative, readingstrategies, and comprehension difficulties. It is found that deductive reasoningis more significantly correlated to expository text comprehension than inductivereasoning. There is other significant differences between natural sciences and socialsciences students in expository text comprehension and deductive reasoning,where the average scores of the natural sciences students on both variables arehigher than those of social sciences students. It shows that the ability to analisesyntesise,to relate some basic elements of a text, and to draw a conclusion ormain idea of the text support the process of expository text comprehension,and can be taught and developed by learning process. Meanwhile, inductivereasoning ability is significantly different between male and female students,and it does not correlate to expository text comprehension.