Mode Division Multiplexing / De-Multiplexing (MD-MUX/De-MUX) is currently investigated as an effective and attractive technique for increasing channel capacity in optical communication networks. The core of such mode multiplexing is usually based on an optical mode converter. Different integrated optical structures have been proposed for mode conversion in planer technology. One of the difficulties in this direction is the design of a mode converter for higher order fiber modes where the energy is distributed in the two transverse dimensions of the guide cross section. In this work, we make use of the two dimensional multi-mode interferencein 2D multimode waveguide (2D MMI) for building a mode converter to convert a fundamental mode LP<sub>10</sub> of single mode fiber to the third order LP<sub>21</sub> mode in multimode fiber. The operating principle of the structure is based on using the 2D MMI with a proper length to create the 4 images on the SM input fiber into the output of the 2D symmetric MMI. The 4 images are distributed in the space in both the x and y directions as a 2x2 matrix form. For these images to form the field distribution of the fiber LP21 mode, a proper phase shift should be associated with each image. This is done using a section of phase shifters based on the control of the waveguide width that allows controlling its relative propagation constant. The proposed design is tested using the 2D Beam Propagation Method BPM. The obtained performance is quite encouraging.