Errors are almost inseparable from learning process, including learning new languages. Though, it is not always considered as negative, instead, it is a part of learning process. This study is conducted to assess students’ mastery of grammar in writing, and to find what grammatical errors they mostly made. In this study, 24 students from English Education Department of UMP were used as samples. The grammatical errors were identified under the theory of Politzer-Ramirez which classify errors into two types: morphological and syntactic error. In the end of the study, twenty-seven types of grammatical errors were identified as the common grammatical errors made by the students. Nine types belong to morphological error, while the remaining fall into syntactic error. Those errors are: the use of prepositions (75 %), redundancy (75 %), simple past tense incorrect (62.5 %), determiners (58.3 %), number confusion (45.8 %), omission of verb (45.8 %), verb construction (41.67 %), derivatives/wrong part of speech (41.67 %), nominalization (33.3 %), the use of pronouns (29.2 %), word order (25 %), past participle incorrect (20.8 %), agreement of subject and verb (16.67 %), misuse of ordinal/cardinal number (16.67 %), third person singular verb incorrect (12.5 %), comparative adjective/adverb incorrect (12.5 %), there transformation (12.5 %), sentence structure (12.5 %), misuse of superlative (12.5 %), incomplete phrase (12.5 %), problems with conjunction (12.5 %), simple present incorrect (12.5 %), prepositional phrase as subject (8.3 %), parallel construction (4.2 %), indefinite article incorrect (4.2 %), the use of cleft sentence (4.2 %), and disagreement with uncountable noun (4.2 %). In morphology level, simple past tense incorrect shares the highest percentage (62.5 %), while indefinite article incorrect and disagreement with uncountable noun shares the lowest (4.2 %). Whereas in syntax level, errors in the use of preposition and redundancy shares the highest percentage (75 %) and the lowest is error the use of cleft sentence (4.2 %).