Mammalian mitochondrial electron transport chain complexes are the most important and complicated protein machinery in mitochondria. Although this system has been studied for more than a century, its composition and molecular mechanism are still largely unknown. Here we report the high-resolution cryo-electron microscopy (Cryo-EM) structures of porcine respiratory chain megacomplex-Ⅰ₂Ⅲ₂Ⅳ₂ (MCI₂Ⅲ₂Ⅳ₂) in five different conformations, including State 1, State 2, Mid 1, Mid 2, and Mid 3. High-resolution Cryo-EM imaging, combined with super-resolution gated stimulated emission depletion microscopy (gSTED), strongly supports the formation of MCI₂Ⅲ₂Ⅳ₂ in live cells. Each MCI₂Ⅲ₂Ⅳ₂ structure contains 141 subunits (70 different kinds of peptides, 2.9 MDa) in total with 240 transmembrane helices. The mutual influence among CI, CIII, and CIV shown in the MCI₂Ⅲ₂Ⅳ₂ structure suggests this megacomplex could act as an integral unit in electron transfer and proton pumping. The conformational changes from different states suggest a plausible regulatory mechanism for the MCI₂Ⅲ₂Ⅳ₂ activation/deactivation process.