Changes of viscoelasticity and structure of water were studied for xanthan (XA)/water systems in annealing and subsequent cooling induced gelation process. Viscoelasticity of XA/water systems was investigated by parallel plate type rheometer. The storage modulus G' of 2 wt.% XA aqueous solution slightly increased with the progression of the annealing time when the solution was annealed at 40 degrees C. Immediately after annealing at 40 degrees C for 18 h, G' of the solution was measured at the gelation temperature T=5 degrees C, G' of the annealed solution was remarkably larger than that of the non-annealed solution. The time evolution of the G' of the system annealed at 40 degrees C for 48 h was measured at T=5 degrees C. G' of non-annealed solution was maintained at constant values at 5 degrees C, in contrast with the G' of the annealed solution increased with increasing gelation time. Structural change of water in the system was measured by differential scanning calorimetry (DSC). The amount of nonfreezing water in the system annealed at 40 degrees C for 24 h changed oscillationally, whereas that of the system annealed at 40 degrees C for 1 h remained almost constant.