The measurements of ultrasonic velocity (υ), density (ρ) and viscosity (η) of the binary liquid mixture of N, NDimethyle acetamide and Benzonitrile of pure and binary mixture for the entire range of compositions at temperature 308 K have been carried out. The experimental data of these mixtures is used to evaluate other thermo dynamical parameters with their excess values such as compressibility (βα), molecular free length (Lf), acoustic impedance (Z), free volume (Vf), internal pressure (πi) and the excess values as excess compressibility ba E, free length Lf E, acoustical impedance ZE, free volume Vf E and internal pressure πi E. From the nature of these excess parameters the nature and strength of the interactions in these binary systems are discussed in view of molecular interaction. The non linearity observed in the plots of these ultrasonic parameters and their excess values with the composition range indicates presence of the intermolecular interaction between the components molecules of the mixture. The nature of excess values of the ba E, Lf E, ZE, Vf E and πi E conforms about the existence of the molecular association between the components of the mixture.