K-Ar age can be determined by means of isotope dilution method with mass-spectrometer MM-1200. Suitable amounts of S0Ar spike- range from 10-11 to 10-12 mole. A standard biotite ZBH-25 is used for the calibration of 38Ar spike. Initial value of 38Ar spike and its standard deviation are calculated by least-squre method. Approximate weights of samples (40Ar/38Ar=1) are listed by computer according to known potassium contents and appoxiraate age. W (mg) can be obtained from that table. when proper (40Ar/38Ar)m is chosen, the appoximate weight of a sample is (40Ar/38Ar)m·W (mg). Estimated precision for angle age measurements is calculated by using a fozmula. Estimated standard deviation at of the spike calibration is ± 0.6%, obtained from the experiments of 38Ar spike calibration. On the basis of repli- cate experiments ak is estimated. When K2O%<0.1%，σk=±1.3%. when O.1%