In this paper, a novel engineering oriented decoupling control system design method for two-input, two-output processes is presented. By employing the concept of integrated error, gain and phase changes of a transfer function when other loops are closed can be uniquely determined. Consequently, an equivalent transfer function matrix for a closed-loop control system can be obtained and its relations with the original process transfer function matrix are derived. On the basis of the equivalent transfer function matrix, parameters of a stable, proper, and causal ideal diagonal decoupler can then be easily determined by proper parameter assingnments of the decoupled transfer function matrix elements. The method avoids the drawbacks of existing decoupling schemes, is very simple and can be easily understood, and can be implemented by field control engineers. An industrial process is employed to demonstrate its simplicity in design and effectiveness in control system performance.