Ion suppression or matrix effect has been widely concerned because of its influence on the characterization of electrospray ionization mass spectrometry. A clear understanding about the ion suppression effect of basic nitrogen compounds in heavy oils is helpful to obtain comprehensive information on the compositions. In this work, the ion suppression effects of basic nitrogen compounds in a processed vacuum gas oil (VGO) and the isolated basic nitrogen fraction were studied under different ionization conditions. The most abundant compounds in the VGO and the isolated basic nitrogen fractions were in the form of N-1 and N-2 species. The influences of the concentrations of the basic nitrogen fractions and formic acids were investigated, which were key factors of ion suppression. The ion suppression effect of the isolated basic nitrogen fraction was evaluated by comparing the nitrogen content obtained by mass spectrometry and elemental analysis. Deviations of less than 30% of the nitrogen content were defined as less suppression conditions, while deviations of more than 30% were defined as more suppression conditions. The low basic nitrogen fraction content and high formic acid content were helpful to reduce the ion suppression effect. It was also found that N-1 species with a low double bond equivalent (such as cyclic amines) were more inclined to ionize under the more suppression condition.