Hydrocarbon compound type analysis by mass spectrometry is a very powerful tool for researchers to develop a petroleum process. A magnetic sector type mass spectrometer with all-glass heated inlet system (AGHIS) has been used for hydrocarbon type analysis since the 1960s. However, the magnetic sector mass spectrometer with AGHIS is already out of date and has difficulties in instrument maintenance. A gas chromatograph equipped with a mass selective detector (GC-MSD) is a very common and more convenient instrument for compound analysis of hydrocarbon mixtures. In this work, we have examined the replacement of the old analysis system with GC-MSD for lube base oil, unconverted oil (UCO) from a hydrocracking process. Indeed, hydrocarbon compound type analysis results of GC-MSD showed a good correlation with those of AGHIS magnetic sector type mass spectrometer as well as better reproducibility. Hydrocarbon compound type distributions were derived from fractions of the total ion chromatogram (TIC) because the GC elutes hydrocarbon mixtures in the order of boiling temperatures. The hydrocarbon compound type distributions of vacumn-distilled fractions of an UCO stock were compared with those deduced from the TIC fractions of the origin UCO sample. The comparison was also illustrated as the R-2 of 0.998. Therefore, GC-MSD is a very efficient and economic instrument for the hydrocarbon compound type analysis and can replace AGHIS magnetic sector type mass spectrometer.