This study reported the grammatical errors made by final year students in STBA Prayoga. The errors were investigated through students' theses writing as their final project before graduate. This study aimed to determine the students' grammatical errors made by the final year students of STBA Prayoga in writing their theses, specifically the morphological errors. The data were gotten from 12 randomly selected students' backgrounds of theses. The type of the research used was descriptive research with documentary technique in collecting data. The data collected were simplified and transformed from raw data into manageable form. After that, the data were classified based on indicators of morphological error proposed by James (2014). Based on the findings, it was shown that not all errors were found on final year students' theses writing. The errors detected in sequence were in using articles (58 errors), using prepositions (48 errors), using plural forms (44 errors), using concord (37 errors), using correct verbs (24 errors), using correct noun (22 errors), using apostrophe (4 errors), andusing correct adjective (3 errors). No error in using adverb was identified on the study. By detecting grammatical errors through the types of errors, educators can see the difficulties experienced by students and help overcome these obstacles by choosing strategies and innovative teaching methods that are appropriate to solve the problems of grammatical errors students face. The study's findings are then recommended as input to help Indonesian learners understand the target language, which in this case is English.