The present invention relates to an epitope of HN protein in Newcastle disease virus which can be recognized by an avian immune system and an antibody against the epitope, a method for detecting a Newcastle disease virus by using the antibody, and an antigenic variant of Newcastle disease virus carrying changes in the epitope. The epitope of HN protein and the antigenic variant of Newcastle disease virus can be used for developing efficient vaccines, and further, in diagnosing the Newcastle disease virus rapidly and exactly.