The adsorption isotherms for N2 over zeolite samples with various structure types or various crystal sizes have been investigated systematically. It is interesting to note that the powder samples of zeolites with cavity structures show oscillation phenomena in their adsorption isotherms on logarithmic x-axis, but the samples without cavities do not show this phenomenon. With increase in the size of the zeolite crystals, the oscillation phenomenon becomes more remarkable. If crystals are large enough and have complex channel structures, even if they do not have cavities, they also exhibit a slight oscillation phenomenon. These results are very helpful to pre-determine which model should be applied for a new sample with an unknown structure, and to get more reliable data on its pore size distribution when Horvath–Kawazoe equation is used to calculate its pore size distribution.