Ensiled green tea waste (GTW) was offered to growing cattle as a protein supplement compared to lucerne hay cube. Rumen fermentation was evaluated by in vivo rumen pH, volatile fatty acids (VFA) and NH₃-N as well as in vitro gas production from hay, maize, and soyabean meal. GTW contained about 340 g/kg crude protein (CP), 90 g/kg total extractable tannins and 14 g/kg condensed tannins, and was offered as protein supplement to growing cattle. GTW silage made up 56 g/kg DM and 130 g/kg CP of the diet in this performance trial. Gas production from maize incubated with rumen fluid of cattle fed GTW was reduced compared with cattle fed lucerne hay cube during 2 months. However, VFA and NH₃-N concentrations of in vivo rumen were not affected and growth performance of cattle was similar to lucerne hay cube treatment. It is concluded that GTW silage could be used as protein supplement in a low proportion (about 50 g/kg DM and 130 g/kg CP of diet) in the ration for growing cattle.