  We measured the in-plane resistivity anisotropy in the underdoped
Ca$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals. The anisotropy (indicated by
$\rho_{\rm b} - \rho_{\rm a}$) appears below a temperature well above magnetic
transition temperature $T_{\rm N}$, being positive ($\rho_{\rm b} - \rho_{\rm
a} > 0$) as $x\leq$ 0.14. With increasing the doping level to $x$ = 0.19, an
intersection between $\rho_{\rm b}$ and $\rho_{\rm a}$ is observed upon
cooling, with $\rho_{\rm b} - \rho_{\rm a} < 0$ at low-temperature deep inside
a magnetically ordered state, while $\rho_{\rm b} - \rho_{\rm a}> 0$ at high
temperature. Subsequently, further increase of hole concentration leads to a
negative anisotropy $\rho_{\rm b} - \rho_{\rm a} < 0$ in the whole temperature
range. These results manifest that the anisotropic behavior of resistivity in
the magnetically ordered state depends strongly on the competition of the
contributions from different mechanisms, and the competition between the two
contributions results in a complicated evolution of the anisotropy of in-plane
resistivity with doping level.
