Optimal design of shallow foundations used in supporting various machinery is of vital importance. Badly designed foundations result in serious problems such as soil failure, large soil settlement, low vibration efficiency and resonance of structures near by the foundation. An optimal design approach based on using MATLAB optimization tool box is used to provide the optimal design of a machinery foundation on a silty soil. To minimize the foundation cost the foundation mass is used as an objective function. To control the high technical quality of the foundation from soil mechanics and vibrations point of view, 10 functional constraints are used. The soil isolation efficiency of the soil is responsible about the vibration transmitted to the surrounding. Therefore, the effect of the minimum isolation efficiency in the range of 71 – 81 % on the foundation optimal design is investigated in details. The performance of the foundation against the minimum isolation efficiency is outlined so that the civil and mechanical engineers can compromise between cost and performance.