The aims of the research were to find out the students’ level competence of derivational suffix in word classes and to know what type of derivational suffix that mostly become the problem for the students. The research was carried out in July 2014 at English Department of Muhammadiyah University of Purwokerto. The population of the research was the second semester students which consisted of 109 students. The researcher used random sampling by taking 32 students as the sample. The method of the research was descriptive method and instruments used in this research were completion and multiple choice test. Based on the result, it was found that students’ competence of derivational suffix in word classes was bad. It can be seen from the mean percentage of students answer: 50% (bad category). Then from the types of derivational suffix forming that were given in the test, the researcher got the percentage of incorrect answer for each derivational forming were as follows; The percentage of noun forming suffix were 53.69%, 75%, 25%. The percentage of verb forming suffix were 67.19%, 50%. Then the percentage of adjective forming suffix were 38.17, 53.12%. The last the percentage of adverb forming suffix was 25%. Based on the explanation it could be concluded that noun forming suffix became the most difficult type of derivational suffix for the students.