This study is an attempt to furnish some helpful data for the maximum use of online information retrieval systems based on the comparative analysis of commands and operating terms employed by DIALOG, ORBIT and BRS which are three largest information retrieval systems in the world. To begin with, the historical development, service contents and retrieval functions of three systems were overviewed in the second chapter, on the basis of which the concrete functions and contents of the commands and the operating terms of the three systems were compared and analyzed one another. Commands were explained divided into basic and special ones. The basic commands subdivided into 44 items were put into the form of a diagram(Table 1). The special commands were first subdivided into search commnad, output-related commands and su n.0, pplementary commands, and then 14 items of which were comparatively analyzed and were represented as a chart(Table 2). In addition, any stand-alone commands employed by each system were also analyzed as a chart(Table 3). The Operating terms other than commands were subdivided into 16 items and were represented as a chart(Table 4). Password, inverted file, search step number in search session, etc were explained to the very detailed extent. A self-evident, single word is that the through understanding of commands is essential to the maximm use of all the functions of each of three systems.