Abstract: Assuming the total volume of immiscible liquid mixture to be the sum of volumes of individual constituents, a new formula has been derived, which relates the effective nonlinearity parameter of mixture (B/A)_eff with parameters of constituents such as nonlinearity parameter (B/A)_i, adiabatic compressibility β_i and their volume fraction X_i. Modified Thermodynamic Method and Finite Amplitude Method are used to measure (B/A)_eff of mixtures. Experimental results agree well with theory.