Abstract: These last years, there is a growing interest for developing tools to the robust control of systems with delays, among these tools there is the Smith predictor the structure of this controller was suggested for the control of the industrial processes with delays, like: Steel factories, transmission lines� etc, but the idea can be generalize for all the processes which have long delays among which: the hydraulic systems with free arias. This study proposes to compare the contribution in the control robustness between the traditional Smith predictor and a modified Smith predictor based in the use of the H8 norm. The satisfaction of the robust and performances condition will be showed and the robustness margins will be compared through an application of the two techniques in the control of a dam river system and also show what the modified Smith predictor can bring in the point of view of stability and performances.