In this work, we study the odd excitation of symmetric multimode interference (MMI) integrated optical structures. We develop a simple formula for the imaging length in the guide under odd excitation in the MMI structures using the symmetry of the structure. The obtained analytical results are verified by numerical calculations using the beam propagation method. The developed model is applied for both 2D and 3D MMI structures successfully. It also allows building functional devices such as power splitters or mode converters for the odd/asymmetric modes of the waveguide.