Abstract: The measurements of acoustic parameters are indispensable in quality control of underwater acoustics systems like sonar. The "Source Level - SL" is a good parameter for sonar's quality evaluations among majority of active sonars. However, this measurement is difficult to use in field conditions, particularly for sonars having narrow beam transducers. The measurement of such a systems require stable conditions, which are hard to achieve in the sea. The paper presents the new method of source level measurement for the narrow beam sonars in the field conditions. The measurement set consists of the reflecting corner and the hydrophon. The set is anchored at selected depth, which may be controlled. The narrow beam transducer is placed on the rotary device, which is mounted on the stable pier or other hydrotechnical constructions. The results of SL measurements are presented for side looking sonar's transducers with 1 deg and 4 deg beams. The analysis of the measurement accuracy was also carried out.