Pulse echo method for nonlinear ultrasound measurement attracts the interest of the scientific community because it can be used for non destructive evaluation and in vivo measurement. Also, it can be useful in quantitative harmonic imaging. The technique is based on the measurement of the reflected harmonics from an interface separating two mediums. The use of this method for nonlinear ultrasound parameter measurement was suggested. However the mathematical model derived was based on the plane wave theory ignoring diffraction phenomenon. An analytical model taking into account diffraction is proposed in this paper. Using a decomposition of the source function in terms of Gaussian beams, analytical expressions are obtained for the fundamental and second harmonic reflected. For the experimental study a ring transducer is used for transmission allowing machining a tube to insert the hydrophone. The hydrophone is a PZT with 0.4 mm in diameter. The harmonics reflected from two interfaces (ethanol/water and water/glycerol) are measured. Comparing the measured values to the theoretical results allow to deduce the nonlinear parameter for water and ethanol. The results obtained agree with the value find in the literature and show the validity of the model.