  Let $\mathcal A$ be a von Neumann algebra and $\mathcal M$ be a Banach
$\mathcal A-$module. It is shown that for every homomorphisms $\sigma, \tau$ on
$\mathcal A$, every bounded linear map $f:\mathcal A\to \mathcal M$ with
property that $f(p^2)=\sigma(p)f(p)+f(p)\tau(p)$ for every projection $p$ in
$\mathcal A$ is a $(\sigma,\tau)-$derivation. Also, it is shown that a bounded
linear map $f:\mathcal A \to \mathcal M $ which satisfies $f(ab)=
\sigma(a)f(b)+f(a)\tau(b)$ for all $a,b\in \mathcal A$ with $ab=S$, is a
$(\sigma,\tau)-$ derivation if $\tau(S)$ is left invertible for fixed $S$.
