We report a new optomechanical pressure sensor using multimode interference (MM) couplers with polymer waveguides and a thin p(+)-Si membrane. We have simulated device characteristics by the normal mode theory and the mode propagation method applied to deflection and strain of the membrane. The optical waveguide is made of a single-mode, rib type of NOA73/PMMA/SiO2 multi-layer system The devices have been fabricated on the thin p(+)-Si membranes which are selectively etched using bulb: micromachining. Device size is 0.4 mm (width) x 13 mm (length) and the total thickness of the membrane is 7 mu m. The device characteristics are measured using a He-Ne laser (lambda = 632.8 nm) as a light source. High sensitivity of 8.2 ppm/Pa has been obtained in the range of 100 kPa. (C) 2000 Elsevier Science S.A. All rights reserved.