Ultrasound has been widely used in the drying, Welding, cleaning, chemical, and heat transfer processes. Moreover, in recent years there has been an increased interest in the application of ultrasound for heat transfer enhancement in the field of heat transfer. However, only a few papers have investigated the effect of ultrasound on heat transfer inside the horizontal tube. Therefore, the objective of this experimental study is to investigate the influence of ultrasound on the heat transfer inside the horizontal tube in the laminar region. In this study, a stainless steel test tube with a diameter of 4 mm was used as the test section under the uniform wall heat flux boundary condition. The entrance and fully developed regions heat transfer coefficients were analyzed. The Reynolds number for the experiments ranged from 600 to 3000. A series of experiments with the different numbers of ultrasonic heads and the different locations of the heads placed on the tube were conducted. The results showed that a substantial enhancement in heat transfer by ultrasound was observed in the laminar region. Based on the combinations of different numbers and positions of ultrasonic heads, two ultrasonic heads were observed to give a better heat transfer enhancement in the entrance and fully developed regions. (C) 2016 Elsevier Ltd. All rights reserved.