Polarized neutron powder diffraction and high resolution powder neutron diffraction measurements with the unpolarized beam were performed in an attempt to determine the low temperature magnetic and crystal structure of CsFeS 2 . The results of the measurements reveal that the magnetic and the crystal structure of the low temperature phase as well as the transition behavior depend strongly on the state of the internal stress of the sample. Using a powder sample of the stress free as-grown fine crystallites, we determine the low temperature crystal structure as a triclinic one with lattice parameters at T =40 K as a =7.08 A, b =12.05 A, c =5.47 A, α=96.6°, β=100.9°, γ=88.5°. The results also suggest that the observed transition may be viewed as that of the Spin-Peierls singlet to the Neel state, which accompanies the simultaneous structural transformation.