A new fitting method has been developed and applied to the plasmoid observations of the GEOTAIL satellite. This method, which we call ''soft fitting'', is regarded as one kind of least square method, and can determine both the fitting function and fitting parameters simultaneously. Its application to magnetic field observations by the GEOTAIL satellite gives an average shape of plasmoid Bz bipolar structures. The method can also provide information about the velocity and spatial size of plasmoids from the optimal fitting parameters. The result indicates that the velocity and size become larger as a plasmoid propagates tailward. ¿ American Geophysical Union 1995