Some samples of the (KNO3)1–х/(CsNO3)x composite with different x values have been prepared, and their temperature dependences of the differential thermal analysis signal, of the dielectric constant, and the amplitude of the third harmonic (to find an existence domain of the polar phase) were studied. The sample surfaces were investigated by scanning electron microscopy. An increase in the proportion of CsNO3 was revealed to lead to a decrease in the coefficient of nonlinearity of the composite and to narrowing of the existence domain’s temperature range of the KNO3 ferroelectric phase III. Also it was found that the composite properties nonlinearity at x beyond 0.5 was determined by the CsNO3 properties.