Writing is very important in developing in the English language, because it is one of the language skills to be mastered. The importance of this skill helps the students in their academic and the social life. Most of the students’ writing ability of the College of Technology is not in a sufficient level. They make many mistakes in their writing. The objective of this research was to identify the knowledge of using punctuation marks in writing and to find out the errors that the students are making in different marks in writing. This research applied descriptive qualitative and quantitative method. The researcher has selected the students at the College of Technology (academic year 2017 which consists of about 2000 students) – Galle as the population of this research. The sample of this research was the students at the first semester of the DCS students. The researcher conducted the study of punctuation errors made by the students. Out of the total number of sixty four students of DCS in 2017, fifty students were selected as the sample using the random sampling method. To collect the data, a questionnaire was given to the teachers of English language to get the idea of the teachers’ perception towards the students’ punctuation errors. A written test was administered to the students to understand how far the students were able to place punctuation marks correctly. First, the researcher held the pre-test to get an idea about students’ knowledge of using punctuation marks and then gave instructions on selected punctuation marks. Finally, done the post test to check whether the students have understood or not. Punctuations that are presented in this research are full stop (period), comma, colon, semicolon, question mark, and apostrophe. More findings revealed that students perform better when they deal with punctuation marks they commonly used in writing such as full stop and comma. However, they are unable to deal with semicolon and colon and sometimes, they neglect some punctuation marks such as apostrophe, question marks when applying them practically. Results from the post test revealed that if students received additional practice and explanation, they will improve their performance when applying appropriate punctuation marks in writing.