We numerically demonstrate the generation of femtosecond rectangular optical pulses by nonlinear directional couplers. Single and cascaded coupler structures are investigated and the conditions for generating a rectangular pulse are presented. It is shown that under realizable conditions, femtosecond rectangular-like pulse can be generated in a second-order cascaded NLDC with 5 mm half-beat length and picosecond ideal rectangular pulse can be achieved in a third-order cascaded NLDC. With respect to the short pulse propagation, the influence of intermodal and third-order dispersions are included in the numerical analysis. © 2005 Elsevier B.V. All rights reserved.