In this study, it is aimed to determine the vocabularies and vocabulary coefficients in the written languages of A1, A2, B1, and B2 students who learn Turkish as a foreign language in line with the frequency of occurrence and prevalence. Structured with a case study by qualitative research, the study was designed with an embedded single-case type. The study group consists of 20 students studying in the TOMER (Turkish and Foreign Languages Research and Application Center) unit of a foundation university. Student's texts in the writing section of the final exam at the end of each level constitute the study object of the research. As a result of the analyses, a total of 3754 vocabulary elements, 742 of them being different, were identified at the A1 level; a total of 3316 vocabulary elements, 662 of them being different, at A2 level; a total of 4464 vocabulary elements, 835 of them being different, at the B1 level; and a total of 1000 vocabulary elements, 520 of them being different, were identified at the B2 level. The vocabulary coefficient was determined as 0.20 for A1, 0.20 for A2, 0.19 for B1, 0.20 for B2 and 0.11 for all levels. The number of vocabulary elements with a prevalence of 4 was determined as a total of 9730, 179 of them being different; the number of elements with a prevalence of 3 was total of 2185, 180 different; and the number of elements with a prevalence of 2 was total of 1956, 361 different. The number of vocabulary elements used in only one level is 2683 in total and 1261 of them are different. The number of new words, which were used for the first time, is 375 in A2, 362 in B1, and 502 in B2. Vocabulary development is 51% in A2, 32% in B1, and 34% in B2. A limitedness was detected in students' use of reduplication, proverb, formulaic expression compared to the use of words, idioms.