In this study, the nonlinear pressure fields of various sized rectangular transducers are modeled in three dimensions. The model was based on Aanonsen's model, which has earlier been presented for circular sources. The nonlinear wave propagation at finite amplitude is presented for square and rectangular type sources up to three harmonics. Numerical results are also tested using available experimental measurements on the acoustic axis. Good agreement is shown with the experiment for both square and rectangular cases. (E-mail: firstname.lastname@example.org) (C) 2006 World Federation for Ultrasound in Medicine & Biology.