It was commonly thought difficult to study argon isotopic fractionation because of affection of radiogenic 40Ar. But, the mass difference(Δm) between 40Ar and 36Ar is 4 and the fractional mass difference(Δm/m)is 1/10, which is larger than that of 13C and 12C(Δm/m=1/13), whose isotopic fractionation is common in nature. Fractionation may arise in the non equilibrium migration of material from one reservoir to another because of the migration rate difference between 40Ar and 36Ar. On basis of this, the authors design a set of experimental system to study the variation of argon isotopic ratio during diffusive migration. Experimental results reveal that the trend of decline of 40Ar/36Ar ratio value occurs when argon diffuses through compact path, that means, if argon diffuses compact path as molecular flow, argon isotopic fractionation can occur because of diffusive rate difference. There are two stages during this procession. At the first stage, degree of fractionation stepwise becomes larger; after this stage, degree of fractionation becomes less because two reservoirs approach to equilibrium. By means of argon isotopic fractionation, we can interpret the feature of argon isotope distribution and distinguish the direction of oil and gas migration, which can enhance oil and gas prospect evaluation and geochemical exploration. Even so, it may provide more important geological information such as origin of magma.