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The high-gain effect of sliding mode control based on VSS (Variable Structure System) suppresses the uncertainties due to parametric variations, external disturbances and variable payloads. The resulting system is completely robust whereas the obtained control law is simple and easy to be applied to on-line computer control. In this paper, the sliding mode controller for MIMO (Multi-Input Multi-Output) robot arm is realized using "Estimated Inertia Matrix" which has inevitably small uncertainties dependent on arm structure and inaccuracies of computation. This technique contributes to practical design of sliding mode for MIMO system compared with the well-known "Hierarchy Method". In addition, simple nonlinear compensation and proper continuous function (instead of relay type component) with "Integral Mode" are implemented to diminish chattering caused by switching input. The validity of this control is confirmed in simulations and experiments where the system shows the robust performances in spite of the existing nonlinear interactions and unknown parametric changes.