The study aimed at investigating the English collocational knowledge of Jordanian graduate students and analyzing quantitatively and qualitatively the collocational errors they made. The sample of the study involved thirty M.A graduate students at the Hashemite university in Jordan. The researcher adapted a completion test that measured students’ knowledge of four types of lexical collocations: free combination, restricted collocations, figurative idioms, and pure idioms. The results showed that free combination created the least amount of difficulty, whereas pure idioms were the most challenging. Additionally, students had unsatisfactory performance on restricted and figurative idioms. In general, the students' deviant answers demonstrated their insufficient knowledge of English collocations. It is concluded that their errors can mainly be attributed to negative first language transfer.