Objective · To investigate the clinical and genetic characteristics of adult late-onset glycogen storage disease type Ⅱ (GSD Ⅱ). Methods · The clinical data, muscular magnetic resonance imaging (MRI) and muscle pathological characteristics of a patient with late-onset GSD Ⅱ were retrospectively analyzed and his family history was investigated. Peripheral blood DNAs of the proband and his father were extracted for genetic testing. Results · The proband presented progressive limb weakness for 7 years and short of breath after activities for 2 years. His parents were inbred, but he was the only one among the 18 members of 5 generations who had clinical phenotype. The blood creatine kinase moderately increased. The muscle MRI of both lower limbs showed that muscles were severely atrophied, and the fat component increased significantly. The pathological findings of muscle biopsy indicated vacuolar degeneration and increased activity of acid phosphatase. Gene sequencing confirmed that the pathogenicity was a homozygous mutation of acid α-glucosidase (GAA) gene c.1634C>T, and his father was a heterozygous mutation of c.1634C>T. Conclusion · Homozygous mutations of GAA gene c.1634C>T may lead to late-onset GSD Ⅱ .