In terms of relative gain and relative frequency, the effective transfer function for independent controller design for multi-input multi-output processes is provided in this paper. Differing from existing equivalent transfer functions, the proposed effective transfer function provides both gain and phase information for decentralized controller design in a simple and straightforward manner. The interaction effects for a particular loop from all other closed loops are directly incorporated into the effective transfer functions in four ways. Consequently, the decentralized controllers can be independently designed by employing the single loop tuning techniques. This design method is simple, straightforward, easy to understand and implement by field engineers. Several multivariable industrial processes with different interaction modes are employed to demonstrate the effectiveness and simplicity of the method. (C) 2006 Elsevier Ltd. All rights reserved.