This research was conducted at SMA Kartika X - 1 on the tenth Grade students aiming of finding out the most typical errors and the causes of error which are made by the students in writing. Based on the aim of the research, the writer uses a descriptive method. In collecting the data the writer administers a kind of writing test to students. Then, the writer analyzes the writing errors done by the students. The result of the research revealed that common errors on the students? writing ability were the error of capitalization ( 20.2%), errors in omit a word ( 15.1%), errors in singular ? plural ( 10.5%), errors in word choice ( 10.5%), errors in verb tense ( 8.4%), errors in add a word ( 8.4%), errors in spelling ( 5.5%), errors in punctuation (4.6%), errors in word order ( 2.9%), errors in meaning not clear (2.1%), article ( 1.3%), errors in run ? on sentence ( 0%). The result of the research revealed that common errors on the sources of error were in interlingual (12.5%), overgeneralization (17.7%), ignorance of rule restrictions (56.7%), incomplete application of rule (0%), and false concept hypothesized (13.5%). It indicates that most of students need more treatment and remedial teaching from the teacher toward their errors in English writings and the tenth grade students of SMA Kartika X - 1 still need more practice in order to develop their writings.