Key words : Content Based Instruction Method, Writing Descriptive Paragraph. The purpose of this research is to increase the students' ability in writing descriptive paragraph through Content Based Instrucion (CBI) method at the Eighth Class of SMP Swasta BNKP Hilimaziaya. Writing is one of the important skill in learning English because through writing, someone can express his/her ideas, feelings, beliefs, or thoughts. Therefore, the students are expected that they are able to write particularly descriptive paragraph. However, based on the researcher's observation in the field, the researcher finds that the students ability in writing descriptive paragraph is low. This is proven by the students average mark that is 55 in which it does not achieve the KKM KD (KKM KD 65). This fact is caused by some problems that should be overcome as soon as possible is teacher's method is not suitable with the students. The researcher method used in this research is Classroom Action Research(CAR) that consist of four procedures, they are planning, action, observation, and reflection. This research is conducted in two cycles and each cycle consist of one meeting. As the collaborative method, in this research, the researcher is helped by an observer that is an English teacher to observe the students and also the researcher activities during teaching and learning process. Based on the research findings, it is found that in cycle I, there are 15 students active (51,72%) and the students who are creative are 19 students (66,51%). The average of the students' mark are 47,41. In cycle II, there are 25 students (86,20%) who are active and the students who are creative are 27 students (93,10%), and the students averages mark is 91,37. Based on the result, it can be conclud that Content Based Instruction (CBI) method can increase the students ability in writing descriptive paragraph.