For the design of individual instructional treatment, the roles played by learning styles on teaching/ learning process were discussed. The research method was not based on the model of a traditional experimental design, but by action research approaches for analyzing performance occurring in real world-learning process. Subjects were undergraduate students majoring in engineering; the learning style inventory developed by Okamoto, T.(1979) was used in order to investigate their learning styles. The main purposes of the present study were to specify the following three points. 1) To survey and arrange theoretical and practical studies on learning styles. 2) To analyze CAI-learning behaviors through three parameters (m, τ, t_o) of Weibull distribution and to estimate learning styles. 3) To examine and discuss the relation between learning styles and learning strategies in problem solving situations. The main results of this research were as follows. 1) On the estimation of three parameters (m, τ, t_o) of Weibull distribution, the values of m and τ were the largest in the group of holist, observing reflectiveness, and then trial-and-error practice. The descending order of the to value was also in observing reflectiveness, holist, and trialand-error practice. 2) In the behaviors of solving the state-space problem in regard to network-graph, the qualitative validity of this conceptual definition of learning styles was verified.