The flow is of great importance for human life in general, and for any industrial process in particular. That is why from early history up to the present day, various methods have been examined to measure the flow as accurately and as efficiently as possible. One of the more recent and modern methods is the ultrasonic method. Ultrasonic flow measurement is performed by means of ultrasonic waves which are transmitted to and from ultrasonic meter sensors. Since ultrasonic meters have no movable parts, they are very easy to install and therefore widely used. However, they should be operated with great care because even a slightest error may cause deviations in their readings. Operations of ultrasonic meters are based on the two main methods: the Doppler method or the transit time method. The meters are also distinguished according to these methods. The ultrasonic meter sensors can be mounted on the outside of a pipe (“clamp-on”) or on its inside (“inline”). The water flow in the penstock of the Rijeka hydroelectric power plant (HE Rijeka) was measured by an ultrasonic meter using the transit time method. The metering was performed from the moment the turbine was put into operation until its shutdown. During the metering, three characteristic stages of the hydroelectric power plant’s operation were monitored: the starting stage, the steady-state operation stage and the shutdown stage. Upon the analysis of the readings and the design of the graphs, the conclusion was reached that the reading data showed some oscillations and deviations from the computer-calculated flow. This was probably due to an error during the installation.