The relation between the crystal size of barium‐strontium oxide and its thermionic emission was studied with the same vacuum tube. The crystal size was measured by an x‐ray diffraction method. The measurements were made in vacuum by using thin mica windows as the entrance and exit ports for the x‐ray beam. The emission was measured with pulse in order to avoid gas from the anode. Below 1050°C the crystal grows slowly, but above it crystal growth is rapid. It seemed that impurity activation is dominant at low temperatures, but heat activation is dominant at high temperatures.