This Research aim to know the retrieval effectiveness on Online Union Catalog of UGM Library, index pattern at data bases, knowing which's more effective of searching at entri oftitle and subjec and also to know the cause ofeffectiveness difference retrieval at the both entri. Along with growth and information accretion, the main problem have shifted from way of accessing information become to chosen the relevant information with its requirement. Information retrieval is not possible to be done in the manual system, because very big information corps and non-stoped
 growing larger. The information retrieval system is very needed to to assist the consumer in finding information. One of the information retrieval system is online union catalog. With the online union catalog, by users easily can look for the book title, pickings of research and other documentation of library where from and any time.
 
 Getting a number of relevant document with the requirement represent the crux in the searching activity, and effectiveness from online union catalog is this represent the key. Effectiveness from a system influenced by a lot of component which each other related/relevant like: quality of input
 metadata in data bases, index, searching strategy, ability of system application and keyword election. A system told effective if the system can find more amount document/ appropriate information of request by precission /high accuracy.
 
 Research done by testing search at online union catalog to use the natural language keyword. Keyword inclusion done at entri of title and entri subjek. Keyword of taken as sampel come from one of title matakuliah of each; every faculty in UGM. From each; every the topic then translated /formulated into 4 natural language keyword. Data obtained is grouped by relevant level. From the acquirement data; then analysed with the test ofnonparametrik Mann Whitney.
 
 From processing data got by conclusion that ratio of retrieval at title entries of equal to 66,6% and subject entries of equal to 58,3%. Data got by mean at title entries of equal to 85,9 document with the detail: very relevant 42,4%, less be relevant 24,2% irrelevant and also 33,5%. While at subject entries found by a mean data of equal to 62,5 document with the detail: very relevant 31,9%, less be relevant 26,45 and irrelevant 30,6%.
 Pursuant to this research result is suggested to increase performance from Online Union Catalog of UGM library. Is things required to improved by technique ofsearching and ability of searching system, the importance of taking care ofquality ofdata input, similarity ofmetadata and always the existence of renewal /updating ofcatalogue data from all member library.
 
 Keywords: effectiveness encounter ofretrieval information retrieval system online union catalog.