In this study, the cognitive characteristics of students' decision-making processes centered on phases, difficulties, and strategies are analysed in the personal dailylife context involving biological knowledge. The subjects were first year science and general high school students in Seoul, Korea; 6 female students and 7 male students. The students' decision-making processes were analysed by "think-aloud" and participant observation methods. On the whole, the students' decision-making processes progressed in following order: recognizing a problem, searching for alternatives, evaluating the alternatives, and decision. During the decision-making processes, the above phases were repeated by trial and error. Students preferred noncompensatory rules that did not allow trade offs among alternatives for decisions, rather than compensatory rules of selection. Students had a tendency to have difficulties in analysing the difference between initial state and desirable state of the problem, organising biological knowledge-related problems, and clarifying values as selective criteria. Even students who had high achievement and more positive science-related attitudes did not apply biological knowledge to search for alternatives, and could not utilise scientific values as selective criteria very well. We discuss the implications of these results for teaching of decision-making in respect to scientific literacy.