Biogenesis of microRNA (miRNA) involves multiple maturation steps. The nuclear RNase III, Drosha initiates the maturation process by cleaving a primary miRNA (pri-miRNA) to release an ~70 nt hairpin-shaped RNA (pre-miRNA). Human Dicer binds to the pre-miRNA with a preference for the 2 nt 3’ overhang. Dicer measures 22 nt from the 5’ phosphorylated end of pre-miRNA and cleaves near the terminal loop. The resulting small RNA duplex is loaded on to Argonaute and one of the strands is selected to form an active RNA-induced silencing complex (RISC). The let-7 miRNA family is highly conserved throughout bilaterian animals. Let-7 miRNAs suppress cell proliferation and promote cell differentiation by targeting multiple genes including HMGA2, RAS, and Lin28. In humans, nine distinct let-7 members are generated from 12 different precursors. Three let-7 sisters (let-7a-2, c, and e) are predicted to carry a typical end structure (2 nt 3’ overhang) as seen in most other premiRNAs outside the let-7 family, which are referred to ‘‘group I”. On the other hand, the precursors of nine let-7 miRNAs (let-7a-1, a-3, b, d, f-1, f-2, g, i, and miR-98) have a 1 nt 3’ overhang. This unusual class is named as ‘‘group II’’. In order to understand the correlation between the structure and/or dynamics of pri-miRNAs and their length of 3’ overhang, we performed NMR hydrogen exchange experiments on the model RNAs of primary let-7a-1 (group II) and let-7c (group I). Comparison of structural and hydrogen exchange data between two group pri-miRNAs provides insights into the molecular mechanism of the cleavage of pri-miRNA by Drosha to produce pre-miRNA.