The　design　of　an　ultra-compact　SOI-based　1×2　3　dB　multimode　interference　(MMI)　coupler　is　proposed.　The　symmetric　multimode　interference　structure　and　deeply-etched　parabolically　tapered　multimode　waveguide　are　used　in　the　device.　The　length　of　the　device　is　reduced　by　about　87%　compared　with　general　multimode　interference　straight　couplers.　Based　on　effective　index　method　(EIM)　and　two　dimensional　beam　propagation　method　(2D-BPM),　the　structure　parameters　are　optimized　and　the　fabrication　tolerances　are　analyzed.　The　results　indicate　that　the　performance　of　the　coupler　is　deeply　affected　by　the　coupler＇s　width,　etching　depth　and　photolithography　alignment　error　in　perpendicular　direction.　©　2009　Tianjin　University　of　Technology　and　Springer　Berlin　Heidelberg.