Abstract: Due to their rigorousness in logical terms and complicatedness in structural relations, long sentences of English for Science and Technology tend to present great difficulties in the course of EST reading and EST translation.Rhetorical Structure Theory is characteristic of analyzing rhetorical relations for semantic coherence of written monologue texts and therefore is widely applicable in text generation and structural analysis.On the basis of the description of RST framework, the present study is concerned with the analysis of long sentences in EST text by means of RST Tool, in an attempt to provide a new analytical tool for structural analysis of EST texts.Our research finds that RST Tool is easily operational, providing significant implications for EST reading and EST translation.