In recent years, there has been a growing trend of using informal styles in academic writing, including research articles. To examine the degree of formality in students' writing, this corpus-based study aimed to analyze the formal linguistic features in the academic writing assignments of English-major students at a Thai university. The learner corpus consisted of 552 assignments, totaling 190,506 words, and was organized into five different writing patterns. TagAnt was used to identify the part of speech for each word, while the Google Colab program was utilized for frequency counting. To assess the level of formality of the corpus, the F-score method proposed by Heylighen and Dewaele (1999) was applied. The results revealed that nouns were the most frequently used formal linguistic feature in student's essays. Despite this, the formality score of the learner corpus ranged from 51 to 53 across five years, indicating a relatively low level of formality. This score suggests that the students' writing was only slightly more formal, signaling the need for further development in grammatical complexity to achieve higher formality in their work. The results highlight the ongoing need for EFL teachers to instruct learners on formal linguistic features in academic writing.