New additional X-ray diffraction layers were observed from KI induced iota-carrageenan, being interpreted:1) as a screw displacement from exact half-staggered double-helix. 2) as a difference between the two chains resulting conformational perturbation. As Nal wt. ratio in iota-carrageenan solution increases, the 1^st and the 5^st layer line get stronger, but the 6th layer line becomes weaker According to X-ray diffraction observation, Cl^- or Br^- ions with a counterpart metallic cation in aqueous solution give almost the same result as I^- ions. It seems that the structural changes depend on the relative difference of hydration energy between counteranions and countercations in solution. Upon the addition of NaI and Ki In fiber state reversible change was detected by X-ray diffraction with performing diffusion-in and-out techniques. Reversible conformation change supports a double-helix model for iota-carrageenans. Therefore, the structural changes of 2iota-carrageenan implicate to a conformational transition from a flexible molecule to a stiff molecule showing no topological constraint and gelation can be conside red in helical level by side-by-side stacking.