[en] The result of preliminary experiment on fuel pin deformation under cyclic power operation are described. (1) A slight rod flattering of 30μm is observed in surface profilometry. (2) Diametral contraction is 10μm. (3) Increasement of temperature during constant power made 0.03mm length change (in appearance) to the rod. (4) In the last stage of cyclic power operation elongation is 120μm at low power level and 80μm (both in appearance) at high power level. Deformation pattern is similar between low and high power levels. (5) By metallographic observation, cracks are not parallel to the axial direction. Those reaching to pellet-pellet boundary are random in direction. Observed crack has branched. (auth.)