  By a model of coupled phase oscillators, we show analytically how
synchronization in {\em non-identical} complex networks can be enhanced by
introducing a proper gradient into the couplings. It is found that, by pointing
the gradient from the large-degree to the small-degree nodes on each link,
increasing the gradient strength will bring forward the {\em onset} of network
synchronization monotonically, and, under the same gradient strength,
heterogeneous networks are more synchronizable than homogeneous networks. These
findings are verified by extensive simulations.
