Abstract: This study investigated into the grammar errors made by seventy students chosen from both first and second year classes of Higher National Diploma in English (HNDE) at Advanced Technological Institute (ATI), Anuradhapura in Sri Lanka. An essay written by each participant on a personalized topic was taken into account in identifying their grammar errors. The errors were categorized under three processes: omission, addition and substitution. Later, their errors were analyzed and the sources of these errors were discussed with emphasis on intralingual and interlingual errors. The findings indicate that omission is the process that recorded the highest number of errors, and both groups (first year and second year) have made similar errors.