Abstract: Grain-Size Analysis Expert System for Sedimentary Rock consistS of knowledge base,inference engine, data base, acquiring knowledge, user interface and daal pre-handling.According tO the reserch result about grain-size analysis for sedimentary rock in recent years,we have stored the knowledge and experience in the Knowledse Base in IF-THEN rules.There are 210 rules in the knowledge base.They can be classified into two typeS, which are used to definename of sedimentary rock in 80 rules, and determine its environment and factes in other rules.Its inference engine that was provided by Expert Develoment tool Vp-Expert nas backward,forward chainining and method of confidence factor.It can automatically choose the knowledge in rules, then records the conclusion and informes the user, or queries or explains about the question to the user during consultation process.In order to acquire a lot of knowledge and experience about gum-size analysis for sedimentary rock from user and make it more flexible, dBASE files or Foxbase files are used as its information base.By means of accessing and inputing daal stored in the information base, we have designed some programs that can actually modify data during consultation process.Therefore, this system has the ability to learn from experience or acquire knowledge from user during consultation process.The model of data pre-handling can be used to calculate some statistical P8rameters and dtaw some statistical graphs of grain-size analysis results.In brief, this system has better user interface, and can handle about data, define itS name and distinguish one from 7 different sedimentary environment or factes when sedimentary rock was formed according to results of grain-size analysis for sedimentary rock.