AlN/GaN superlattices (SLs) have been subject of intense investigation due to their potential applications for high power transistors and ultraviolet laser diodes. However, only a few reports are focused to their structural properties. In this work we report on the extended study of strained AlN/GaN SLs. Utrathin (0.42/0.48 nm) AlN/GaN SLs were grown by the MOCVD technique on the top of a structure composed of thick 1000 nm Al_xGa_(1-x)N buffer layer, and AlN nucleation layer (≈20nm) on the vicinal sapphire (0001) substrates. These samples were analyzed by means of high resolution x-ray diffraction (HRXRD) techniques, and cross-sectional transmission electron microscopy (TEM). It was found that for Al_xGa_(1-x)N layers with the same Al content as the average Al content in the whole volume of AlN/GaN SLs exhibit a good in-wafer composition uniformity. .Moreover, no relaxation was observed i.e. both AlN and GaN layers have the same in-plane lattice parameter as the underlying buffer layer. The buffer layer is hence an effective substrate for the two-dimensional growth and ensures the proper the chemical and crystalline order of the AlN/GaN SLs. The strain evolution in the SLs structures is assessed by the high-resolution x-ray diffraction and reciprocal space mapping (RSM), using numerical simulation methods. It has also been shown that x-ray methods are sensitive enough to provide data necessary to determine chemical composition profile and crystallographic order even in the case of extremely thin AlN and GaN layers.