In this paper, for the purpose of testing the ultrasonic flow meter simultaneous flow rate measurement using ultrasonic measurement method and gravimetric measurement method was conducted using the gravimetric measuring line located in the Laboratory for Process Measurement of the Faculty of Mechanical Engineering and Naval Architecture in Zagreb. Firstly, as an introduction, brief definitions of volume and mass flow rate, with a short overview of various flow meters, were given. Secondly, after the introduction, the principle of ultrasonic measurement method of volume flow rate was described in more detail. Two ultrasonic measurement methods commonly used in the industry, transit time method and the frequency shift method, were also described. Following, the principle of electromagnetic measurement method was described. After the overview of those two methods, gravimetric measurement was also decribed, including brief explanations of static and dynamic methods. Considering that the testing of ultrasonic flow meter was performed in the Laboratory for Process Measurement, descriptions of measuring lines located in the said laboratory were given as well. Furthermore, as the subject of this paper, the ultrasonic flow meter was described with stated recommendations for the installation. After the theoretical part, the preparation and implementation of simultaneous ultrasonic and gravimetric measurements were given. The measurement results were shown in tables and graphs, and the standard uncertainty using type A and type B evaluation methods was calculated. In the end, the expanded uncertainties for the results of both methods were calculated and shown in tables and graphs.