In this master's thesis a reference block for calibration of an ultrasonic system is designed. In the introduction, non destructive methods with detailed description of the ultrasonic method and echo technique are listed. Advantages and disadvantages of the echo technique and the description of ultrasonic waves are given. Equipment for ultrasonic testing is presented, along with a short description of basic parts of an ultrasonic device with a special reference to an angular ultrasonic probe. In addition to the mode of operation of the angular probe, the phenomenon of ultrasonic beam refraction between two materials (Snell's law) is explained with the calculation of first and second critical angle of the chosen angular probe. The zones of the ultrasonic beam and the method of calculating the basic parameters of it are listed. In addition to the systematization of standards that are most widely applicable for the calibration of the ultrasonic system, the calibration of ultrasonic test system using a known surface of the reference reflectors is developed, with stated estimation of the size of the reference reflector using DGS diagrams is stated. The reference block designed for calibration of an ultrasonic system is shown in detail with an explanation of its additive manufacturing. With the elaboration of a technical documentation of the reference block, the phases of additive technology are listed. The CT scan of the reference block has been made in a Laboratory for Precision Length measurements (LFSB) at the Faculty of Mechanical Engineering and Naval Architecture with the aim of obtaining more accurate dimensional values of the constructed reference reflectors. The final part of this thesis (with conclusion) presents an estimate of the size of the reflector with an ultrasonic system using the DGS diagram, and then the estimate via the ultrasonic device.