The effects of AlN interlayer thickness on the transport properties of nearly lattice-matched InAlN/GaN heterostructures grown on sapphire substrates by pulsed metal organic chemical vapor deposition have been studied in detail. A very high electron mobility of approximately 1425 cm(2)/V s at room temperature and 5308 cm(2)/V s at 77 K together with a two dimensional electron gas (2DEG) density of 1.75 x 10(13) cm(-2) were obtained for nearly lattice-matched InAlN/GaN heterostructures with an optimum similar to 1.2 nm thick AlN interlayer. For comparison, InAlN/GaN heterostructure without AlN interlayer exhibited a 2DEG density of 1.61 x 10(13) cm(-2) with low electron mobility of 949 and 2032 cm(2)/V s at room temperature and 77 K, respectively. This significant enhancement of electron mobility is mainly attributed to an optimized AlN interlayer, which provides a smooth interface between InAlN barrier layer and GaN buffer layer and hence remarkably reduces the alloy disorder scattering by suppressing carrier penetration from the GaN channel into the InAlN barrier layer. Simultaneously, a best surface morphology with a root mean square roughness value of 0.24 nm is obtained with the optimized AlN interlayer. (C) 2012 Elsevier B.V. All rights reserved.