This research aimed to find out the most frequent types of inflectional suffixes, derivational suffixes, noun phrases and verb phrases produced by the Postgraduate Students in writing argumentative text. This research used Descriptive Quantitative Research. The data were obtained by using writing test. The samples of this research were 16 Postgraduate Students of English Education Master Program in Academic Year 2023 selected by using Total Sampling Technique. The results of data analysis from 16 samples found that 1) The inflectional suffixes of plurality {-s} were 46,06%, 3rd person singular {s/es} were 8.33%, past tense/passive {-ed} were 14.58%, participle {-ing} were 17.59 %, possessive {-s} were 3,93%, and comparative/ superlative degrees {er/est} were 3.47%. 2) The derivational suffixes forming noun were 52.4%, forming verb were 3.2%, forming adjective were 23.92%, and forming adverb were 19.28%. 3) The constructions of simple noun phrases were 18.86%, rather complex noun phrases were 23.58 %, and very complex noun phrases were 56.83%. 4) The constructions of simple verb phrases were 82.5% and rather complex verb phrases were 16.87%. Thus, it can be concluded that the most frequent types of inflectional suffixes used by the students were plurality {-s}, the type of derivational suffixes was noun formation, the type of noun phrases was very complex noun phrases, and the type of verb phrases was simple verb phrases.