All complete monosomy 21 appear to be lethal early in their development in humans and only survive in mosaic forms. Complete monosomy 21 is a very rare and usually debilitating genetic disorder. Partial monosomy 21 is also rare and is thought to constitute a clinical syndrome consisting of peculiar faces, hypertonia, psychomotor retardation, and slow growth. We experienced a case of monosomy 21 mosaicism. Chromosome analysis demonstrated mosaicism for cell lines in the lymphocytes examined; 45, XX, -21/46, XX. The main clinical features were craniofacial dysmorphism including high arched palate, submucosal cleft, micrognathia and arthrogryposis-like symptoms including flexion deformity of fingers. And hematological findings were revealed dyserythropoiesis, thrombocytopenia and eosinophilia. Currently, the patient has nearly compatible growth, but a mild degree of mental retardation. We report here an 8 years old female child with apparent monosomy 21 mosaicism associated with dyserythropoiesis, thrombocytopenia and eosinophilia, with a review of the associated literatures.