This historical comparative study is concerned with a group of three languages in Nusa Tenggara (Southeast Lesser Sunda islands), Indonesia, namely Balinese, Sasak, and Sumbawa. These three languages are hypothesized to have a close genetic relationship.The result of the present study shows that the precentage of similarity among the three languages, based on Swadesh 200-Wordlist is qualitatively 50%. This level of similarity is higher than the similarity to Javanese and Madurese, that is 36%, or td Bima and Manggarai which 31%. On the lower level, the similarity between Sasak and Sumbawa is 64%.By the qualitative approach it was also found the lexical and phonological innovation were found. The phonological innovation that supports the grouping is the presence of mutual metathesis, while lexical innovation comprises 41 words. Phonological innovations that separate the group are the loss of the first consonant (C1) in consonant clusters in word medial position; nasal assimilation and the change of *R > r are found in Sasak and Sumbawa. In Balinese *R > O/#-, *R > 0 (V-V), and *R > h/-#. 31 lexical innovation were found in Sasak and Sumbawa.Based on the quantitative and qualitative approaches that are supporting each other, it is concluded that the three languages namely Balinese, Sasak, and Sumbawa belong to one.