The effects of the weak magnetic field on the negative ion (H-) extraction in a negative ion source have been studied by means of a two-dimensional electrostatic particle simulation. A particle-in-cell model is used which simulates the motion of the charged particles in their self-consistent electric field. In addition, the effect of the electron diffusion across the weak magnetic field is taken into account by a simple random-walk model with a step length Δx per time step Δt; where D⊥ and ξx are the perpendicular diffusion coefficient and normal random numbers. Under this simple diffusion model, the electron diffusion has no significant effects on the H- transport. Most electrons are magnetized by the weak magnetic field and lost along the field line. As a result, more H- ions arrive instead of electrons in the region close to the plasma grid in order to ensure the plasma neutrality. © 2004 American Institute of Physics.