  External pressure driven phase transitions of FeSe are predicted using
\textit{ab initio} calculations. The calculations reveal that $\alpha$-FeSe
takes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from
NiAs-type to MnP-type and CsCl-type FeSe is also predicted. MnP-type FeSe is
also found to be able to transform to CsCl-type FeSe, which is easier from
$\alpha$-FeSe than the transition to MnP-type FeSe, but comparable to the
transition from NiAs-type FeSe. The calculated electronic structures show that
all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds
becomes stronger and the covalent interaction becomes weaker when the
structural phase transition occurs from $\alpha$-FeSe to the other phases of
FeSe. The experimentally observed decrease in $T_{c}$ of superconducting
$\alpha$-FeSe at high pressure may be due to a structural/magnetic instability,
which exists at high pressure. The results suggest us to increase the $T_{c}$
of $\alpha$-FeSe if such phase transitions are frustrated by suitable methods.
