Abstrak Penelitian ini berupaya mengukur kualitas kosakata bahasa Indonesia berdasarkan perhitungan keragaman dan kekerapan leksikal pada siswa kelas XII SMKS Bina Karya 2 Karawang. Pengukuran kualitas penguasaan kosakata merupakan alternatif untuk menilai kapasitas dan pemahaman tata bahasa antarsiswa. Penelitian ini menggunakan pendekatan kualitatif dengan metode deskriptif dan melibatkan 40 peserta didik kelas XII SMKS Bina 2 Karawang sebagai subjek penelitian. Objeknya adalah analisis kualitas kosakata yang dinilai berdasarkan keragaman dan kekerapan leksikal dari teks karangan novel sejarah pribadi. Teknik pengumpulan data yang diaplikasikan di antaranya adalah observasi, wawancara semistruktur, dan tes membuat novel sejarah pribadi. Hasil penelitian, yaitu (1) Kualitas penguasaan kosakata ditinjau dari keragaman dan kekerapan leksikal yang diperoleh siswa kelas XII SMKS Bina Karya 2 Karawang masih rendah, (2) Persentase keragaman leksikal tertinggi diperoleh siswa Rendi Wijaya yaitu 66.67%, sedangkan persentase Karya terendah diperoleh siswa Angga yaitu 40.96%. (3) Persentase kekerapan leksikal tertinggi diperoleh siswa Sahdi yaitu 47.87%, sedangkan persentase terendah diperoleh siswa Angga yaitu 31.36%. (4) Kata ‘saya’ paling banyak muncul dalam teks karangan siswa kelas XII karena sudut pandang orang pertama umumnya digunakan dalam topik sejarah pribadi, (5) Jenis kata benda mempunyai persentase terbanyak karena topik tentang sejarah pribadi lebih cenderung merujuk pada orang, tempat, benda, peristiwa, atau hal lain yang akan membentuk serangkaian pengalaman. Selain itu, kata benda juga merupakan titik acuan dalam kalimat. Variasi kualitas penguasaan kosakata siswa dipengaruhi faktor psikologis, fisiologis, dan lingkungan.Kata Kunci: penguasaan kosakata, keragaman, kekerapan, leksikal AbstractThis study attempts to measure the quality of Indonesian vocabulary based on the calculation of lexical diversity and frequency in class XII students of SMKS Bina Karya 2 Karawang. Measuring the quality of vocabulary mastery is one of the alternatives to assess the capacity and understanding of grammar among students. This study used a qualitative approach with descriptive method and involved 40 students of class XII SMKS Bina Karya 2 Karawang as research subjects. The object of this research is the analysis of vocabulary quality assessed based on lexical diversity and frequency of personal history novel essay texts. The data collection techniques used include observation, semi-structured interviews, and a test to create a personal history novel. The results of the study, namely (1) The quality of vocabulary mastery in terms of lexical diversity and frequency obtained by students of class XII SMKS Bina Karya 2 Karawang is still low, (2) The highest percentage of lexical diversity is obtained by student Rendi Wijaya, namely 66.67%, while the lowest percentage is obtained by student Angga, namely 40.96%. (3) The highest percentage of lexical frequency is obtained by Sahdi students, namely 47.87%, while the lowest percentage is obtained by Angga students, namely 31.36%. (4) The word ‘I am’ appears the most in the essay texts of grade XII students because the first person point of view is generally used on personal history topics. (5) Noun types have the highest percentage because topics about personal history are more likely to refer to people, places, things, events, or other things that will form a series of experiences. In addition, nouns are also reference points in sentences. Variations in the quality of students' vocabulary mastery are influenced by psychological, physiological, and environmental factors.Keywords: vocabulary mastery, diversity, frequency, lexical