Mass of open cluster NGC 7789 in the Galaxy is determined by three methods: the photometric method based on mass-luminosity relations of stars in the cluster Mpho=7712.5M⊙; the dynamical method based on virial theorem Mvir= 6996.1Mo; and the tidal radius method based on the interaction between the cluster and the Galaxy Mtta= 5152.5M⊙. The mean mass of this cluster is estimated to be Mc= 6620.4± 762.5M⊙.