The research is aimed to describe the types of grammaticalerrors, especially the process of forming Arabic nounphrases (NP) done by students of Department of ArabicLanguage and Literature of State University for IslamicStudies Sunan Kalijaga Yogyakarta, and to explain thefactors that influence those errors. The data of this study istaken from the result of writing exam in the class of kitābahmuhakkamah or 'academic formal writing' of the fifthsemester. The method, which is used to describe thegrammatical errors, especially the making process of ArabicNPs, is the method of error analysis. The result of theanalysis shows that the frequency of errors in the process offorming the Arabic NPs, which happens repeatedly, iswhen the patterns of NP are noun+noun (N+N) andnoun+adjective (N+Adj). The error of NP with a pattern ofN+N happens when the article (al-) at head noun is used,while the error of NP with a pattern of N+Adj happenswhen the article (al-) at head noun, which has definitemodifier, is omitted. The factors, which influence themaking errors of Arabic NP, are grammatical factors. Thegrammatical factors are caused by the different rulesbetween the Indonesian and Arabic NP. On the other hand,the process of language acquisition of second or foreignlanguage makes these problems worse.