Laser diodes are widely used in many fields, but the poor beam quality is an obvious deficiency. The intensity distribution of a beam from a laser diode in the slow axis is so complex that it is difficult to be described by a Gaussian distribution of a certain order. The beam qualities of a certain type of laser diode in the directions of perpendicular and parallel to junction are evaluated in this paper. The intensity distributions in two directions are described respectively by fundamental mode Gaussian distribution and multi-mode Hermite-Gaussian distribution. The computed data is basically in accord with the experimental data. The mathematical model may provide some suggestions for the designing of optical system with laser diodes and related simulations.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.