Abstract: Rare earth metal can be chelated to target peptides by using bicyclic anhydride diethylenetriamine-N,N,N’,N’’,N’’’-pentaacetic acid dianhydride (DTPA dianhydride), which can be used as tags and analyzed by matrix-assisted laser desorption/ionizationtime of flight MS (MALDI-TOF MS) for protein quantification. To further improve the labeling efficiency, reaction temperature, pH value, time and the amount of metal tag were optimized in this study. And for the first time, guanidiation was introduced to make the labeling only take place at the N-terminal of the peptide, thus the selectivity was improved. The results showed that the guanidiation rate of all peptides from standard proteins exceeds 95%, and guanidiation had little influence on the labeling. The results from seven standard proteins showed that at least three peptides of each protein can be stably labeled, and can be used for ideal tags for protein quantification. The interference of ion suppressive effect can be corrected by standard curve, and this suppressive effect has no influence on protein quantification before the ratio of two metal ions reached to 6. The improved strategy provides a technology that can be used in protein quantification not only at the peptide level, but also at the protein level.