In this study, we measured the complex efficiency of a physiologically active antibody, a chelator and radiosiotopes using the ESI-TOF/Ms system for develop good radiopharmaceuticals. For a precise measurement, TLC is a low accuracy method. Loading of same amount of sample is difficult for each test, and work to quantify accurately the results obtained through TLC cannot be afforded compared to the use of other analytical instruments. The method of analysis using a mass spectrometer is capable of a mass analysis of proteins for quantitative analysis. The conjugates of the chelator (CHX-A- DTPA) and the antibody (IgG) were separated through MWCO, and were analyzed using ESI-TOF and MALDI-TOF mass spectrometry. The analysis using MALDI-TOF is roughly divided into measurements on mass spectrometry. When conjugating a small molecular weight of CHX-A-DTPA and a large molecular weight of IgG, distinguishing the peak of the conjugate and the peak of IgG was difficult. However, an ESI-TOF mass spectrometer system is capable of an analysis of mass by decentralizing the IgG. It is utilized as a technique for measuring the metabolic processes during conjugation and the stability evaluation of radiopharmaceuticals. When establishing this technique, the accuracy of the overall radiophar-maceutical analysis is expected to be able to be improved.