The paper describe a computer-based method to calculate the series resistance R s of a millimeter wave Ka-band packaged IMPATT diode from small signal conductance-susceptance characteristics. The series resistance R s has been calculated at the threshold condition when the small signal conductance of the packaged diode just becomes negative and the device susceptance becomes positive. Again, the value of series resistance R s has been determined from the measurement of threshold current and threshold frequency with the silicon Ka-band IMPATT diode embedded in a resonant cap cavity which agrees well with the values obtained by the computer method.