  We demonstrate a general procedure to construct entanglement witnesses for
any entangled state. This procedure is based on the trace inequality and a
general form of entanglement witnesses, which is in the form $W=\rho-c_{\rho}
I$, where $\rho$ is a density matrix, $c_{\rho}$ is a non-negative number
related to $\rho$, and $I$ is the identity matrix. The general form of
entanglement witnesses is deduced from Choi-Jamio{\l}kowski isomorphism, that
can be reinterpreted as that all quantum states can be obtained by a maximally
quantum entangled state pass through certain completely positive maps.
Furthermore, we provide the necessary and sufficient condition of the
entanglement witness $W=\rho-c_{\rho}I$ in operation, as well as in theory.
