This study was to investigate a procedure to cognitively assess students\' knowledge states with applying Rule-space theory by answering two questions; how a cognitive model (that is a Q-matrix) can be developed, how the diagnosed results are characterized and different from ability scores. Four competing Q-matrices, using CSAT (College Scholastic Achievement Test in Korea) Mathematics items, were independently developed by two mathematics specialists based on two attribute sets - CSAT Mathematics test specifications and SAT Mathematics attributes previously developed by Boodoo, et. al. (1997). Then item difficulties were regressed to the attribute scores using each Q-matrix. A Q-matrix was finalized by combining attributes having only one item-connection into the one of same category, which was based on the Q-matrix showed the highest R2. Two kinds of knowledge states, attribute attainment pattern and attribute attainment probability pattern, were computed. The findings with diagnosis results were that the difference of conceptualization of knowledge states might result in different diagnoses, which lead to a discussion that two types of diagnoses were to be provided together, and that there were individual differences of knowledge states among the students having same scores, which lead to an importance of cognitive diagnosis in educational assessment settings.