  We performed the high hydrostatic pressure resistivity measurements (up to
1.7 GPa) on the newly discovered superconductors $A_{x}$Fe$_2$Se$_2$ ($A$ = K
and Cs) single crystals. Two batches of single crystals $K_xFe_2Se_2$ with
different transition temperatures ($T_c$) were used to study the effect of
pressure. The $T_c$ of the first one gradually decreases with increasing
pressure from 32.6 K at ambient pressure. While a dome-like behavior was
observed for the crystal with $T_c=31.1$ K, and $T_c$ reaches its maximum value
of 32.7 K at the pressure of 0.48 GPa. It indicates that there exists a optimal
doping with maximum $T_c$ of 32.7K in $K_xFe_2Se_2$ system. The behavior of
$T_c$ vs. pressure for $Cs_xFe_2Se_2$ also shows a dome-like behavior, and
$T_c$ reaches its maximum value of 31.1 K at the pressure of 0.82 GPa. The hump
observed in temperature dependence of resistivity for all the samples tends to
shift to high temperature with increasing pressure. The resistivity hump could
arise from the vacancy of Fe or Se.
