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Human metapneumovirus (hMPV) is an important pathogen that causes upper and lower respiratory tract infections in children worldwide. hMPV has two major genotypes, hMPV-A and hMPV-B.Epidemiological studies have shown that the two hMPV genotypes alternate in predominance worldwide in recent years. Co-circulation of the two genotypes of hMPV was usually observed and there is no study about the interaction between them, such as competitive replication, which maybe the possible mechanisms for alternating prevalence of subtypes. Our present study have used two different genotypes of hMPV (genotype A: NL/1/00; B: NL/1/99) in different proportions in animal model (BALB/c mice) and cell model (Vero-E6) separately. The result showed that the competitive growth does exist in BALB/c mice, genotype B had a strong competitive advantage. However, genotype B did not cause more severe disease than non-predominant (genotype A) or mixed strains in the study, which were evaluated by the body weight, airway hyperresponsiveness and lung pathology of mouse. In cell model, competitive growth and the two genotypes alternately prevalence were observed. In summary, we confirmed that there was a competitive replication between hMPV genotype A and B, and no difference in disease severity caused by the two subtypes. This study shows a new insight to understand the alternation of hMPV genotype prevalence through genotype competition and provide experimental evidence for disease control and vaccine design.Human metapneumovirus (hMPV), which was first isolated from children with respiratory tract infections (RTIs) in the Netherlands in 2001 and belongs to the Paramyxoviridae family and Pneumovirus subfamily 1 , is a leading cause of respiratory infections in children worldwide 2,3 . hMPV has been recognized as a causative agent of respiratory infections, including upper respiratory infections, severe bronchiolitis and pneumonia, in all age groups, with more severe disease occurring in infants, elderly people, and immunocompromised hosts 4-7 . The epidemic season of hMPV is reported to occur from winter to early spring 8 . There are no effective vaccines or drugs to combat hMPV infection.hMPV has been speculated to have similar characteristics as human respiratory syncytial virus (RSV), and this has been gradually confirmed by research over the last 18 years 8-10 . The seasonality, clinical manifestations, and age distribution of hMPV infection resemble those of RSV infection, and the alternating pattern of the two