MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) is a rare congenital mitochondrial DNA mutation disease. Here, we report a 4-year-old girl, who presented with short stature, mental retardation, and recurrent seizures, underwent simultaneous F-FDG PET/MRI examination. An interesting contradiction images were found on bilateral frontal, left temporal, occipital, and parietal lobes, which were with high blood flow shown on 3D-ASL perfusion images, but low uptake of F-FDG on PET images. The contradiction of high blood flow and low glucose metabolism gave us a clue to make the diagnosis of MELAS. The final diagnosis was MELAS confirmed by genetic testing.