This paper describes a study designed to investigate the development of scientific reasoning or logical thinking patterns of South Korean secodary school students. The scientific reasoning or logical thinking patterns were categorized into patterns of logic such as seriation, combinations, proportion, control of variables, probability, and proposition. The investigation intended to determine when South Korean secondary school students use the scientific thinking patterns in problem-solving situations and to identify the level of intellectual development which the students reach in terms of Piagetian developmental stages. The conclusions reached as a result of the study include: (1) the formal thinking variables of combination and two-dimensional reasoning develop relatively early, while proposition and probability develop comparatively late; and (2) the cognitive development level of South Korean secondary students in this study did not coincide with Piaget's prediction, but displayed similar results to some earlier studies. Additional results related to student variables including geographical location, sex, and socio-economic status are also reported. (TW)