Natural C14 measurements have been performed at the Dept. of Physics, National Taiwan University since 1965, using a cylindrical proportional counter of 1000 ml (Hsu et al., 1965). The proportional counter is operated with CO2 as the filling gas at a pressure of 1216 mm Hg at room temperature of 20°C. Working voltage is 4.7 Kv, with a plateau length of more than 700 v and a plateau slope of ca. 1% per 100 v. Background is reduced to 5.1 counts/min and the counting rate of the NBS oxalic acid standard is 9.4 counts/min at the normal counting pressure of 1216 mm Hg.