A glass‐sealed Langmuir probe was developed to get accurate information on the ionospheric plasma. An electrode sealed in a glass tube is first baked in low pressure and tipped off the pumping system to keep the electrode surface clean. An essential advantage of the glass‐sealed Langmuir probe is that it will not require the rapid sweeping of a probe voltage for reliable data acquisition. It is noted that the glass‐sealed Langmuir probe is strongly recommended for the measurement of energy distribution of thermal electrons by the second harmonic method.