Receptor-binding specificity of HA, the major surface glycoprotein of influenza virus, primarily determines the host ranges that the virus can infect. Influenza type B virus almost exclusively infects humans and contributes to the annual "flu" sickness. Here we report the structures of influenza B virus HA in complex with human and avian receptor analogs, respectively. These structures provide a structural basis for the different receptor-binding properties of influenza A and B virus HA molecules and for the ability of influenza B virus HA to distinguish human and avian receptors. The structure of influenza B virus HA with avian receptor analog also reveals how mutations in the region of residues 194 to 196, which are frequently observed in egg-adapted and naturally occurring variants, directly affect the receptor binding of the resultant virus strains. Furthermore, these structures of influenza B virus HA are compared with known structures of influenza A virus HAs, which suggests the role of the residue at 222 as a key and likely a universal determinant for the different binding modes of human receptor analogs by different HA molecules.