The effect of an external magnetic field on a critical point for phase transitions in a dusty plasma is investigated. It is shown that the ambient magnetic field increases the effective hard core radius of dust particles, which, in turn, would affect a critical point in terms of the Coulomb coupling parameter and the ratio between the inter-dust grain spacing and the dusty plasma Debye radius. The present result may be useful in understanding the phenomenon of liquid-vapor phase transitions in laboratory dusty plasmas that are held in an external magnetic field.