  We devise a new quantum key distribution scheme that is more efficient than
the BB84 protocol. By pre-announcing basis, Alice and Bob are more likely to
use the same basis to prepare and measure the qubits, thus achieves a higher
efficiency. The error analysis is revised and its security against any
eavesdropping is proven briefly. Furthermore we show that, compared with the
LCA scheme, our modification can be applied in more quantum channels.
