  We generalize several inequalities involving powers of the numerical radius
for off-diagonal part of $2\times2$ operator matrices of the form
$T=\left[\begin{array}{cc} 0&B, C&0 \end{array}\right]$, where $B, C$ are two
operators. In particular, if $T=\left[\begin{array}{cc} 0&B, C&0
\end{array}\right]$, then we get \begin{align*} {1\over
2^{{3\over2}(r-1)}}\max\{ \| \mu \|, \| \eta \| \} \leq w^{r}(T)\leq
\frac{1}{2^{r+1}} \max\{ \| \mu \|, \| \eta \| \}, \end{align*} where $r\geq 2$
and $ \mu=|(C-B^{*})+i(C+B^{*})|^{r}+|(B^{*}-C)+i(C+B^{*})|^{r}$, $
\eta=|(B-C^{*})+i(B+C^{*})|^{r}+|(C^{*}-B)+i(B+C^{*})|^{r}$.
