Two kinds of glass specimens were prepared in the form of powders and blocks with anorthite compositions, and an examination was made of the sintering and crystallization behaviour of the specimens and the microstructure dependence of their thermal expansion. It was found that the addition of a few wt% of TiO2 was necessary to obtain a good sintered body by heat treatment at 1050-degrees-C. Specimens with low expansion coefficients were obtained when no TiO2 was added and when the grain size of the starting glass powder was large (ca. less than 180 mum). It was estimated that this was mainly due to the presence of orthorhombic and hexagonal anorthite in the central part of each grain. In order to explain the abnormally low expansion coefficient of the sintered specimens, the crystallization behaviour of glass blocks with the same composition was examined. The crystallization proceeded by two mechanisms : the triclinic phase precipitated from the surface, and the orthorhombic and the hexagonal phase precipitated from the central part of the specimens. The orthorhombic phase showed a strong tendency for a preferred orientation, and the thermal-expansion measurements showed strong anisotropy. The addition of TiO2 to the mother glass suppressed the precipitation of the orthorhombic and hexagonal phases, and the resultant pieces were composed mainly of the triclinic phase; the thermal-expansion coefficient became isotropic.