This research focuses on comparing and studying the vocabularies in three inhabited dependent islets of Jeju Island, U-do, Biyang-do and Gapa-do. One on one or one on two interviews with Informants over mid-seventies in these islets were conducted on 304 dialect vocabularies. Just 62 words (21.67%) are commonly used in those three islets and 131 words (including duplicate 135 words, 45.80%) are used in two islets. Different forms of 73 words (25.52% of all categories) are researched in each islet. 59 words (45.03%) in both Biyang and Gapa-do, 37 words (28.24%) in both Gapa and U-do, and 35 words (26.71%) in both U and Biyang-do are commonly used. The result that various forms of dialect vocabulary are found in these three islets has a connection with administrative districts of Goryeo Dynasty, Dongdohyeon and Seodohyeon. Common word forms in Biyang-do and Gapa-do, which used to be part of Seodohyeon, are more than those in U-do, which used to be part of Dongdohyeon, and Biyang-do, and U-do and Gapa-do. Each islet is located separately so language is divided into different forms. That is why 73 different words (25.52%) are used in these small islets. One of the linguistic features of these islets is unique linguistic forms and high frequency of ancient language. There used to be less interchange with the main island, Jeju, before transportation system is developed. Because of the unique culture, different word forms and conservatism of language is shown. Moreover, some of vocabulary found in the main Jeju island is not researched in these small islets due to the influence of the natural and life environment. It can explain that linguistic environment is affected by the ecological environment. In addition, there are 13 words about plants out of 25 words which are not used anymore.