The solid state morphology of the main-chain liquid crystal BB-6 polyester was studied by the small angle X-ray scattering (SAXS) method. The BB-6 polyester forms isotropic, smectic A, and crystal phases in the order of decreasing temperature; thereby its crystallization is taking place from the smectic A phase. The SAXS for the crystalline specimens prepared by cooling the isotropic melt at a rate of 10 degrees C min(-1) shows the well-defined reflection maxima which are attributable to the stacked lamellar structure. The lamellar spacings are distributed around 250 Angstrom so that an appreciable number of chain foldings are included in a chain. The lamellar size is increased by annealing the crystal like in other crystallizable polymers. In contrast, it is not essentially altered by annealing the smectic A phase. The results show that the chain foldings exist at a thermodynamic equilibrium in the smectic A phase. The correlation length between the chain foldings is assumed approximately to be 250 Angstrom, which corresponds to 15 times the length of the repeat unit.