Spelling is one of the important things for students in learning writing. However, in writings, students frequently committed errors in spelling the English words. This research aimed to find out the types of spelling errors and the type of error that frequently occurs in students’ writing. This research used qualitative design and the instrument of this research was document analysis. This research conducted at SMA Muhammadiyah 3 Yogyakarta (senior high school). The researcher selected the participants by using convenience sampling and took 30 students’ hortatory texts from three classes of grade XI of Science Program. The result showed that students class XI Science Program (IPA) at SMA Muhammadiyah 3 Yogyakarta committed ten categories of spelling errors. These ten categories covered omission of a letter, addition of a letter, single letter instead of double letter, double letter instead of single letter, substitution of one letter, interchange of two adjacent letters, involving an apostrophe, erroneous splitting or joining of words (word segmentation error), two or more error of the same type or of different types (multiple error), and sound based error. Meanwhile, the most frequently error that occurred in the students’ writings was single letter instead of double letter category.