This article reports a successful example that an injected sample is simultaneously introduced into two fused silica capillary columns and detected separately by a flame ionization detector (FID) and an HP 5973 mass selective detector (MSD) both installed on an HP 6890 Gas Chromatograph. This GC and GC/MS analyses of aromatic hydrocarbons in oils and rock extracts were carried out by using this method and good results were achieved. This method is of great value in organic geochemistry research because it is efficient and economical.