Abstract: As one component of neutron diffraction spectrometer, Soller neutron slit col limator is applied to collimation of neutron beam. When a collimator is designed, it is needed to choose not only kind and thickness of absorbing material, but also length of collimator for high transmission and small space. Collimator optimization was tried with Monte-Carlo simulation. Optimization was carried out for absorber thickness and length of collimator, and the calculating result was evaluated.