In this study, it is aimed to determine thevocabularies and vocabulary coefficients in the written languages of A1, A2,B1, and B2 students who learn Turkish as a foreign language in line with thefrequency of occurrence and prevalence. Structured with a case study byqualitative research, the study was designed with an embedded single-case type.The study group consists of 20 students studying in the TOMER (Turkish andForeign Languages Research and Application Center) unit of a foundationuniversity. Student's texts in the writing section of the final exam at the endof each level constitute the study object of the research. As a result of theanalyses, a total of 3754 vocabulary elements, 742 of them being different,were identified at the A1 level; a total of 3316 vocabulary elements, 662 ofthem being different, at A2 level; a total of 4464 vocabulary elements, 835 ofthem 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 vocabularycoefficient was determined as 0.20 for A1, 0.20 for A2, 0.19 for B1, 0.20 forB2 and 0.11 for all levels. The number of vocabulary elements with a prevalenceof 4 was determined as a total of 9730, 179 of them being different; the numberof elements with a prevalence of 3 was total of 2185, 180 different; and thenumber of elements with a prevalence of 2 was total of 1956, 361 different. Thenumber of vocabulary elements used in only one level is 2683 in total and 1261of them are different. The number of new words, which were used for the firsttime, is 375 in A2, 362 in B1, and 502 in B2. Vocabulary development is 51% inA2, 32% in B1, and 34% in B2. A limitedness was detected in students' use ofreduplication, proverb, formulaic expression compared to the use of words, idioms.