Abstract: An apparatus which had an extremely small dead volume and could determine intrapellet gas effective diffusivities by chromatography rapidly and accurately at 250℃ was built. The statistical moment theory was introduced and equations describing the relations between the diffusivities and the experimental moment were derived. For a B110 catalyst pellet, (De)Ar-N2 and (De)He-N2 at 29℃ and 1.114 × 105Pa were 0.009344 and 0.02936 cm2/s respectively and its tortuosity factor was equal to 3.16. The results agree well with theoretical calculation. Both the experimental technique and the apparatus are reliable.