Thermal properties of polysaccharide hydrogels formed by physical crosslinking are reviewed. Attention is especially paid for change in gelling mechanism affected by thermal history. The effect of thermal history on gel-sol transition was investigated with highly sensitive differential scanning calorimetry (DSC) in order to clarify the non-equilibrium state of k-carrageenan hydrogels. Thermal histories markedly affect on the junction zone formation. The effect of annealing on xanthan gum molecules was investigated with DSC and atomic force microscopy (AFM). Oscillational change in nonfreezing water content (W_nf) observed with DSC suggests that molecular chains of xanthan gum aggregate and dissociate as a function of time. AFM images provide direct information concerning oscillational change in the network structure. These AFM images accord well with variation of W_nf measured with DSC. Low-methoxyl pectin forms various kinds of hydrogels when divalent cations are added. Calcium-pectin hydrogels were prepared with annealing the sol state of the aqueous solution of pectin. Viscoelastic properties of calcium-pectin hydrogels are measured by thermomechanical analysis (TMA) in water. Dynamic modulus (E' ) of calcium-pectin hydrogels decreases with increasing annealing temperature and time.