The effects of source plasma parameters and space charge neutralization on energy spread of very low energy and high perveance ion beam have been studied experimentally. Plasma parameters of a bucket source were changed by shorting an arc chamber and a positive electrode (a plasma electrode) to deteriorate electron confinement. Although the change of plasma potential reflected the beam energy and the change of energy spread was observed, the relation between the plasma parameters and energy spread is unclear. Space charge neutralization of the ion beam by supplying thermal electron and increasing gas pressure in beam transport region showed reduction of the energy spread. This result suggests that the space charge effects of the ion beam plays an important role in energy broadening. ©1996 American Institute of Physics.