Z39.50 (ISO 23950), is the international standard and protocol for information retrieval. It has been used in the area of information because of the powerful function, interactive operation and contents in abundance.WWW-Z39.50 gateway has inherited and developed the structure of traditional Client/Server. By combining the Z39.50 with WWW, and giving users simple, uniform, friendly user interface, this system colonized the usage of Z39.50 greatly. WWW-Z39.50 gateway is a key technology of “New-generation Library Information System ”. In this article we introduce the structure, function and technology of WWW -Z39. 50 gateway, and compares it with the C/S. In the end, we talk about the new trend of Z39. 50.