The present study was conducted to identify and classify the most common errors committed by students at postgraduate level in English writing skill. English is considered as second language for Pakistani students. It is used as medium of instruction in many universities in Pakistan. The main focus of the study was to investigate L2 learners' errors in writing skill. To achieve the objective of the study, data from 90 students were obtained from six institutes of Khyber Pakhtunkhwa. Fifteen participants were selected randomly from each institute, studying at postgraduate level at the department of English. Sample students were assigned to write essay of 250 to 300 words on one of the three given topics: My Favourite Season, An Unforgettable Incident of my Life, and My Hobby. For identifying common errors in L2 writing, the students' writings were analyzed with the help of checklists in order to classify different types of errors made by the respondents. The results of the study revealed that the most common and frequent errors made by the students were of verb tense, spelling, inappropriate use of vocabulary and subject/verb agreement.