The effect of air exposure on the crystal structure of alkyl substituted organic thin film was studied using in situ grazing incidence X-ray diffraction (GIXD) measurements. The GIXD measurements were carried out by scanning the specific qxy region using a two-dimensional area detector and the Soller slit. The a and b axes of the monoclinic unit cell were found to shrink by air exposure. As for the c axis, there was no detectable change caused by air exposure. The shrinking of the a and b axes immediately occurred after exposure to air, and then saturated in six days. The shrinking of the a and b axes by air exposure might affect the transport properties, because shortening between intermolecular distances should promote the hopping of hole carriers.