In this study, the new Pareto Optimum data base which is the most important module of the expert CAD system for designing rotary tiller shaft was constructed by using the modified method of dynamic programming and the mainframe computer. The results were as follows (①~⑧:performance items); 1)The commercial rotary tiller shaft is designed with minimization of vibration as one of the most important objectives. So the values of ① the dynamic unbalance(Q) and ② the transverse vibration(D) are very low compared with the values in the date base. 2) The date base has many excellent rotary blade arrangements. There are about 17 kinds between blade number 24 to 56, and the data base is better than the commercial rotary shaft from the viewpoint of each of the 5 performances items which are ③ the degree of uniformity of the soil slices(S_d), ④ the degree of flatness of the soil surface(K), ⑤ the fluctuation of torque in cutting soil(T_d),⑥ the required values of the power(T_a), and⑦ the torque difference in fluctuation of the right and left half of the rotary shaft(T_<1rd>). 3)⑥ The required values of the power for the blade arrangements in the data base fluctuate about 10%~20% from the mean value. 4) ⑧ The fluctuation in the values of the required power per 1m width greatly increases for blade arrangements with less than the 40 blades.