Temperature dependence of lattice parameters of bismuth sesquioxide (Bi2O3) has been obtained between 26 degrees and 778 degrees C by the Rietveld method using neutron powder diffraction data. Lattice parameters a(alpha)(*), b(alpha)(*), c(alpha)(*), a(delta) and unit-cell volume of alpha and delta phases increased, while the beta angle of alpha phase decreased with an increase of temperature. Here the a(alpha)(*) denotes the lattice parameter a of the alpha phase on the basis of pseudo-fluorite lattice. The thermal expansion coefficients of alpha phase were 26.7, 6.6, and 9.0 ( x 10(-6) degrees C-1) for a(alpha)(*), b(alpha)(*), and c(alpha)(*) axes, respectively, indicating a large anisotropy. The alpha- to -delta phase transition of Bi2O3 was confirmed between 721 degrees and 760 degrees C on heating. At the alpha-delta transition point, the lattice parameters and unit-cell volume discontinuously changed, indicating that the transition is of the first order.