Experiment was conducted on the threshold pressure for atmospheric air through unconsolidated narrow-size distributed mini-sphere and sand particles at low flow rates. The threshold diameter calculated from measured threshold pressure showed that it did not follow the traditional similarity theory，but also affected by Knp. This is consistent with the authors’experiment on accurate permeability measurement

and can be explained as a result of gas flow slip effect within such micro-pore structure. The current work tends to find the method for predicting the permeability-threshold pressure relationship for unconsolidated porous structures so as to develop a basis for accurately measuring the thermophysical properties in the field.