  The thermal conductivity of iron-based superconductor CsFe$_2$As$_2$ single
crystal ($T_c =$ 1.81 K) was measured down to 50 mK. A significant residual
linear term $\kappa_0/T$ = 1.27 mW K$^{-2}$ cm$^{-1}$ is observed in zero
magnetic field, which is about 1/10 of the normal-state value in upper critical
field $H_{c2}$. In low magnetic field, $\kappa_0/T$ increases rapidly with
field. The overall field dependence of $\kappa_0/T$ for our CsFe$_2$As$_2$
(with residual resistivity $\rho_0$ = 1.80 $\mu\Omega$ cm) lies between the
dirty KFe$_2$As$_2$ (with $\rho_0$ = 3.32 $\mu\Omega$ cm) and the clean
KFe$_2$As$_2$ (with $\rho_0$ = 0.21 $\mu\Omega$ cm). These results strongly
suggest nodal superconducting gap in CsFe$_2$As$_2$, similar to its sister
compound KFe$_2$As$_2$.
