The Drosha–DGCR8 complex is essential for primary microRNA (pri-miRNA) processing. Drosha, a nuclear RNase III enzyme, cleaves pri-miRNAs into hairpin-shaped pre-miRNAs, which are then processed by Dicer into mature miRNAs. While Dicer and other RNase III proteins have been studied extensively, the mechanism of Drosha remains unclear. This study characterizes the action mechanism of human Drosha by generating mutants and identifying its new interacting partner, DGCR8. The basic mechanism of Drosha is similar to that of Dicer, with its RNase III domains forming an intramolecular dimer and cleaving the 3′ and 5′ strands of the stem. Drosha functions as a large complex, interacting with DGCR8, which contains two double-stranded RNA (dsRNA)-binding domains. DGCR8 is essential for pri-miRNA processing, along with Drosha. The study proposes a model for the action mechanism of class II RNase III proteins, showing that Drosha and DGCR8 form a complex that processes pri-miRNAs. The results indicate that Drosha and DGCR8 are critical components of the pri-miRNA processing complex, with DGCR8 playing a role in substrate recognition and complex formation. The study also shows that Drosha forms intramolecular dimers, with each domain cutting a different strand of the pri-miRNA hairpin. The findings highlight the importance of Drosha and DGCR8 in miRNA biogenesis and provide insights into the structural and functional characteristics of the Drosha–DGCR8 complex.The Drosha–DGCR8 complex is essential for primary microRNA (pri-miRNA) processing. Drosha, a nuclear RNase III enzyme, cleaves pri-miRNAs into hairpin-shaped pre-miRNAs, which are then processed by Dicer into mature miRNAs. While Dicer and other RNase III proteins have been studied extensively, the mechanism of Drosha remains unclear. This study characterizes the action mechanism of human Drosha by generating mutants and identifying its new interacting partner, DGCR8. The basic mechanism of Drosha is similar to that of Dicer, with its RNase III domains forming an intramolecular dimer and cleaving the 3′ and 5′ strands of the stem. Drosha functions as a large complex, interacting with DGCR8, which contains two double-stranded RNA (dsRNA)-binding domains. DGCR8 is essential for pri-miRNA processing, along with Drosha. The study proposes a model for the action mechanism of class II RNase III proteins, showing that Drosha and DGCR8 form a complex that processes pri-miRNAs. The results indicate that Drosha and DGCR8 are critical components of the pri-miRNA processing complex, with DGCR8 playing a role in substrate recognition and complex formation. The study also shows that Drosha forms intramolecular dimers, with each domain cutting a different strand of the pri-miRNA hairpin. The findings highlight the importance of Drosha and DGCR8 in miRNA biogenesis and provide insights into the structural and functional characteristics of the Drosha–DGCR8 complex.