Diffuse scattering of ultrasound by the microstructure of polycrystal specimens can be used to evaluate grain size and grain elongation. The existing diffuse scattering models mostly dealt with circular transducers whose symmetrical sound field is insensitive to the asymmetric elongated grain. The sound field of a rectangular transducer provides a new perspective for acquiring additional information. First, the existing single scattering response (SSR) and double scattering response (DSR) models are modified for a rectangular transducer, where the sound field of a rectangular transducer is equivalent to that of an elliptical transducer in the far-field. Therefore, an equivalent single Gaussian beam model is derived using amplitude-equivalent and beamwidth-equivalent coefficients. Then, the spatial correlation function of elongated grains is transformed into the wavenumber domain, giving rise to the SSR and DSR of a rectangular transducer that reveals the interaction effect of an asymmetric sound field and elongated grains on ultrasonic backscattering. The experimental results show that the sizes of elongated grains in a cold-rolled aluminum are evaluated as 1086 ± 8, 90 ± 4, and 10 ± 1 μm in the x, y, and z directions, where the exact values are 1184.2 ± 11.9, 80.7 ± 5.2, and 8.3 ± 0.5 μm according to metallographic measurements.