  We have computed the number of polarization modes of gravitational waves
propagating in the Minkowski background in $f(R)$ gravity. This is three of two
from transverse-traceless tensor modes and one from a massive trace mode, which
confirms the results found in the literature. There is no massless breathing
mode and the massive trace mode corresponds to the Ricci scalar. A newly
defined metric tensor in $f(R)$ gravity satisfies the transverse-traceless (TT)
condition as well as the TT wave equation.
