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Grain boundary character and microstructural evolution during hot deformation of a high Nb-containing TiAl alloy with the composition of Ti-45Al-8.5Nb-(W,B,Y) have been investigated in this work. The plasma arc-melted alloy exhibits nearly lamellar microstructures with β/B2 phase precipitating along colony boundaries. The retained β/B2 phase rich in Ti, Nb and W and poor in Al is believed to be formed by the β → α transformation during nonequilibrium solidification. This β/B2 precipitation is caused by the relative high cooling rate, low atom diffusion rate of β stabilizers and different partition coefficient of the elements. B and Y elements exist mainly as borides and Y2O3, respectively. The morphology, size, composition and stability of β/B2 phase are determined by hot deformation. The high temperature and press enhance the elements diffusion, thus the variation of the composition of β/B2 phase in deformed samples is obviously observed. When the samples are deformed in (α + γ) phase region, the transformation β/B2 → α2 takes place. Considering the different crystal structures between β/B2 and α2 phases, there is a tendency for β/B2 phase to transform to an increased close packed structure of α2 during deformation. The β/B2 → α2 transformation is promoted by the hot deformation.