An multi-objective optimization (MOO) and knowledge extraction were conducted for a structural design of a regional jet horizontal tail using Multi-Objective Design Exploration (MODE). MODE reveals the structure of the design space from the trade-off information and visualizes it as a panorama for a Decision Maker. The present form of MODE consists of Kriging model, Adaptive Range Multi-Objective Genetic Algorithm and Self-Organizing Map (SOM). Combination between the stringer-pitch and the rib-pitch was optimized based on detailed buckling evaluation and MSC/NASTRAN static analysis using realistic aircraft structural model. The resulting Kriging model provided several solutions with improvements, both in the structural weight and the number of structural components, compared with the baseline design. Furthermore, SOM divided the design space into clusters with specific design features. The acquired design knowledge from the present application has been utilized for the horizontal tail design of Mitsubishi Regional Jet (MRJ).