Abstract: The pressure of inert gas in quartz tubes is critical to the performance of high-pressure mercury lamps. The vacuum in high-pressure mercury lamps were determined by gas chromatography-mass spectrometry (GC/MS). From the relative mass calibration factor of argon fm’, and the ratios of peak area of argon to nitrogen (A_Ar/A_N2) were measured by GC/MS. We induced a equation which was described the relation between pressure of P_Ar and (A_Ar/A_N2) or the mass ratios of argon to nitrogen (m_Ar/m_N2). Using this equation with other relative data, P_Ar in the air and high-pressure mercury lamps were measured. The result shows that difference in the pressure of argon among those high-pressure mercury lamps with different performance and type of high-pressure mercury lamps, which are different.