A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 x 10(3) S m(-1) for Bicu(0.975)Seo at 650 degrees C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (similar to 0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 mu V K(-1)), ZT at 650 degrees C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.