  Let $Alg \mathcal{N}$ be a nest algebra associated with the nest $
\mathcal{N}$ on a (real or complex) Banach space $\X$. Suppose that there
exists a non-trivial idempotent $P\in Alg\mathcal{N}$ with range $P(\X) \in
\mathcal{N}$ and $\delta:Alg\mathcal{N} \rightarrow Alg\mathcal{N}$ is a
continuous linear mapping (generalized) left derivable at $P$, i.e.
$\delta(ab)=a\delta(b)+b\delta(a)$
($\delta(ab)=a\delta(b)+b\delta(a)-ba\delta(I)$) for any $a,b\in
Alg\mathcal{N}$ with $ab=P$. we show that $\delta$ is a (generalized) Jordan
left derivation. Moreover, we characterize the strongly operator topology
continuous linear maps $\delta$ on some nest algebra $Alg\mathcal{N}$ with
property that $\delta(P)=2P\delta(P)$ or $\delta(P)=2P\delta(P)-P\delta(I)$
every idempotent $P$ in $Alg\mathcal{N}$.
