Alpha-fetoprotein (AFP) is a well-established serum biomarker for hepatocellular carcinoma (HCC) in clinical laboratories. However, AFP levels can often be high in benign liver diseases such as liver cirrhosis. For this reason, specifically, the level of the aberrant N-glycosylation of AFP has been proposed as a HCC biomarker to improve diagnostic performance using targeted mass spectrometry (MS). In this study, we developed an endoglycosidase-assisted absolute quantification (AQUA) method by which to measure N-glycosylated AFP levels in serum using liquid chromatography-parallel reaction monitoring with immunoprecipitation. Especially, an isotopically labeled synthetic N-glycopeptide with N-acetylhexosamine (HexNAc) attached to asparagine (N) was used as an internal standard. The efficacy of this method was demonstrated by quantifying the N-glycosylation of AFP in human serum. As a result, we showed that the lower limit of the quantification of a stable isotope-labeled N-glycopeptide reached an attomolar level. Our method also had a linear dynamic range from 2 to 6000 ng/mL for N-glycosylated AFP levels. Finally, the N-glycosylation levels of AFP were measured in HCC patients and in healthy donors with the coefficient of variation in both cases (<10% CV). To the best of our knowledge, this is the first report of the AQUA of N-glycosylated AFP in human sera using a stable isotope-labeled glycopeptide as an internal standard. The results demonstrate that our method can facilitate the discovery and verification of aberrant glycoprotein biomarkers in human serum and plasma through sensitive and precise quantification.