  Partial transpose is an important operation for quantifying the entanglement,
here we study the (partial) transpose of any single (two-mode) operators. Using
the Fock-basis expansion, it is found that the transposed operator of an
arbitrary operator can be obtained by replacement of a^{{\dag}}(a) by
a(a^{{\dag}}) instead of c-number within normal ordering form. The transpose of
displacement operator and Wigner operator are studied, from which the relation
of Wigner function, characteristics function and average values such as
covariance matrix are constructed between density operator and transposed
density operator. These observations can be further extended to multi-mode
cases. As applications, the partial transpose of two-mode squeezed operator and
the entanglement of two-mode squeezed vacuum through a laser channel are
considered.
