The magnetic structure in a localized 5f system U₃Pd₂₀Si₆ was studied by means of neutron diffraction. We revealed a remarkable collinear structure and found a new type of spin--flop transition due to the collinear coupling. Spin wave excitation was observed in a whole Brillouin zone. We concluded that the low energy quasi-elastic response is the excitation of the quasi-particles due to hybridization between 5f and conduction electrons.

PACS numbers: 61.12.--q, 75.30.--m