  Recently, the nodal line semimetals have attracted considerable interests in
condensed matter physics. We show that their distinct band structure can be
detected by measuring the collective modes. In particular, we find that the
dependence of the plasmon frequency $\omega_p$ on the electron density $n$
follows a $\omega_p \sim n^{1/4}$ law in the long wavelength limit. Our results
will be useful in the ongoing search for new candidates of nodal line
semimetals.
