The etch of $(Na_{0.5}K_{0.5})NbO_3$ (NKN) thin film was performed in $BCl_3/Cl_2/Ar$ inductively coupled plasma. It was found that the 1sccm addition $BCl_3$ (5%) into $Cl_2/Ar$ plasma caused a non-monotonic behavior of the NKN etch rate. The maximum etch rate of NKN was 95.3 nm/min at $BCl_3$ (1 sccm)/$Cl_2$ (16 sccm)/Ar (4 sccm), 800 W ICP power, 1 Pa pressure and 400 W bias power. The NKN etch rate shows a monotonic behavior a s the bias power increases. The analysis of the narrow scan spectra of XPS for both a s-deposited and etched NKN films allowed one to assume ion assisted etch mechanism. The most probable reason for the maximum etch rate can be defined as a concurrence of chemical and physical etch pathways.