Ultrasound test is a widely used non-destructive method for determining the mechanical and metallurgical properties of materials. In this method, ultrasonic wave velocity or attenuation coefficient is measured and measurement accuracy is very important. In this paper, variations of longitudinal wave velocity are studied in the presence of a thermal gradient both theoretically and numerically using a 2D model. A linear temperature distribution is assumed and the length of the work piece and the temperature of the hot side are considered as varying parameters. A new 2D theoretical model is developed for this problem. The test piece is made of st37 steel. To evaluate the proposed equation, we assume constant temperatures and the length of the work piece are varied in the range of 0.05-0.1 m. Then, we study the effect of the temperature of the hot side from 398 -998 K. By ANSYS software, a novel two-dimensional finite element model (FEM) is developed in axisymmetric state for this problem. The results of the theoretical model are compared with those obtained from the numerical model and a very good agreement is observed.