A new method of determining the cathode temperature of the diode, triode, and multigrid tube, using high harmonic component of initial current which flows to the anode when an ac voltage is applied to the anode, is presented. The principle and apparatus of this method are described, and the causes of error in measurement are discussed. The advantage of this new method is that the temperature may be read directly; the time for measuring is extremely short; the error due to the leakage current is reduced; and it has a high accuracy (within ±0.05 percent) and good reproducibility. Results on diode 6AL5, 6H6, triode 6J6, and pentode 6SH7 taken by this method, and results taken by other method, are compared.