According to the phase diagram, 6Bi2O3.GeO2 composition melts congruently at 93$0^{circ}C$ and forms a stable ${gamma}$-6Bi2O3.GeO2 crystal phase below the melting point. But when the melt of this composition was cooled at a rate 1-15$0^{circ}C$/min without tapping by a glass rod or impurity addition, a metastable $delta$-6Bi2O3.GeO2 crystal phase was formed. It is due to that as the nucleation energy barrier of $delta$-6Bi2O3.GeO2 crystals, which have more open and defective structure, is lower than that of ${gamma}$-6Bi2O3.GeO2 crystals. When impurities or ${gamma}$-6Bi2O3.GeO2 crystals existed in the melt, stable ${gamma}$-6Bi2O3.GeO2 crystal phase was formed at various cooling rate. It is because of that the impurities or the ${gamma}$-6Bi2O3.GeO2 crystals role as a seed crystal and as a result the nucleation energy barrier of ${gamma}$-6Bi2O3.GeO2 crystals is lowered.