Stellar magnetic activity is important for formulating the evolution of the star. To represent the stellar magnetic activity, the S index is defined using the Ca II H+K flux measure from the Mount Wilson Observatory. MgII lines are generated in a manner similar to the formation of Ca II lines, which are more sensitive to weak chromospheric activity. MgII flux data are available from the International Ultraviolet Explorer (IUE). Thus, the main purpose of this study was to analyze the magnetic activity of stars. We used 343 high-resolution IUE spectra of 14 main-sequence G stars to obtain the MgII continuum surface flux and MgII line-core flux around 2,800 ˚A. We calculated S index using the IUE spectra and compared it with the conventional Mount Wilson S index. We found a color (B − V ) dependent association between the S index and the MgII emission line-core flux. Furthermore, we attempted to obtain the magnetic activity cycles of these stars based on the new S index. Unfortunately, this was not successful because the IUE observation interval of approximately 17 years is too short to estimate the magnetic activity cycles of G-type stars, whose cycles may be longer than the 11 year mean activity cycle of the sun.