The purpose of this study is to find out the grammatical errors that the students mostly make in their writing description. This research was carried out 11th may 2009 and 17th may 2009 at SMAN 9 Tangerang. The writer analyzed 20 students at tenth-eight class by making descriptive composition. Qualitative analysis is the technique of data analysis in this research. This analysis starts from collecting the data, correcting the students? errors, classifying errors, and making reconstructions based on the students? error. The data were obtained from writing descriptive test which the writer gave to the students. Meanwhile, the writer found that the most typical error made by the students are word choice with 77 or 31 % errors Then, it is followed by error in capitalization area with 33 or 13% errors, in verb tense area with 24 or 9.6% errors, error in omit a word writer found 16 or 6.4 % errors, in article area the writer found 15 or 6 % errors, errors in word form, add a word, and punctuation are 12 or 4.8 % errors, in meaning not clear and singular plural are 10 or 4%, then error in word order are 9 or 3.6 %, errors in run-on sentence and spelling area are 8 errors or 3.2 %, and errors in incomplete sentence is 4 errors or 1.6 % errors. In accordance to the typical errors above, the writer found that the most sources of error which cause students making the errors are interference of the students? mother tongue, incomplete application of rule, ignorance of rule restriction, overgeneralization, and false concept hypothesized. The result of this study concludes that the students still had problem in vocabulary and they had not understood how to place appropriate words in appropriate sentence. It means that students? ability in making writing is still low.