In this paper, we present the idea of interval valued fuzzy subgroup defined over a certain t-conorm (-IVFSG) and prove that every IVFSG is-IVFSG. We use this ideology to define the concepts of-IVF cosets,-IVFNSG and formulate their various important algebraic characteristics. We also propose the study of the notion of level subgroups of-IVFSG and investigate the condition under which a-IVFS is-IVFSG. Moreover, we extend the study of this phenomenon to introduce the concept of quotient group of a group Z relative to the-IVFNSG and acquire a correspondence between each-IVF(N)SG of a group Z and-IVF(N)SG of its quotient group. Furthermore, we define the index of-IVFSG and establish the-interval valued fuzzification of Lagrange's theorem of any-IVFSG of a finite group Z .