According to the principle of measuring Boltzmann constant based on the ampere-volt characteristic of PN junction，the diode and triode are used as the substitute samples of PN junction in the experiments. The result has shown that the Boltzmann constant measured directly by the diode is larger than the theoretical value which is inconsistent with the results in some literatures. Then the reason is analyzed based on the factors of diffusion current， recombination current and surface current. The Boltzmann constant measured by using the base-level circuit composed of triode is relatively more accurate because of the influence of recombination current and surface current being reduced.