Based on the data obtained from the Spitzer/Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIPMSE) Legacy Program and the Two Micron All Sky Survey (2MASS) project, we derive the extinction in the four IRAC bands, [3.6], [4.5], [5.8], and [8.0] μm, relative to the 2MASS K_s band (at 2.16 μm) for 131 GLIPMSE fields along the Galactic plane within |l| ≤ 65^o, using red giants and red clump giants as tracers. As a whole, the mean extinction in the IRAC bands (normalized to the 2MASS K_s band), , , , , exhibits little variation with wavelength (i.e., the extinction is somewhat flat or gray). This is consistent with previous studies and agrees with that predicted from the standard interstellar grain model for R_V = 5.5 by Weingartner & Draine. As far as individual sightline is concerned, however, the wavelength dependence of the mid-infrared interstellar extinction varies from one sightline to another, suggesting that there may not exist a "universal" IR extinction law. We, for the first time, demonstrate the existence of systematic variations of extinction with Galactic longitude which appears to correlate with the locations of spiral arms as well as with the variation of the far-infrared luminosity of interstellar dust.