This paper deals with the analysis of impact of various factors on power loss and efficiency of worm gearing. Factors that significantly affect the power losses are, principally, types of materials of meshed gears and geometry of worm pair, type and viscosity of lubricating oils, input rotational speed, worm shape, loading, temperature, etc. Results of experimental tests of worm gearing efficiency are also presented in the paper. Tests were conducted according to planned experiment schedule on specialized device AT200, where single-stage worm gearboxes with transmission ratio i=18 has been used. During the test stage, oils of different iscosities (ISOVG 220, ISOVG 460 and ISOVG 680) have been used and their impact on efficiency was monitored. The values of efficiency level are determined at different input rotational speeds (circumferential velocity) and output torques (loads).