The structural and the optical properties of 10-period In0.15Ga0.85N/GaN multiple quantum wells (MQWs) have been investigated using HRXRD (high-resolution X-ray diffraction) and PL (photoluminescence). For the samples, the barrier thickness was kept constant, 7.5 nm and the well thicknesses were varied, 1.5, 3.0, 4.5, and 6.0 nm. For the structural characterization, an ω/2θ-scan and an ω-scan for GaN (00 2) reflection and a reciprocal space mapping (RSM) around the GaN (10 5) lattice point were employed. The average strain for the MQWs increased as the well thickness increased. The MQW with a 6.0 nm well thickness experienced lattice relaxation and the crystallinity of the sample was poor compared to that of the other samples. MQWs with well thicknesses of 1.5, 3.0 and 4.5 nm, however, maintained lattice coherency with the GaN epilayers underneath, and the critical well thickness for lattice relaxation of the MQWs used in the study was 6.0 nm. The PL spectra showed that the relative emission intensity of the sample with a 6.0 nm well thickness was lower than for the others, a fact consistent with the X-ray results. The emission intensity, therefore, is considered to be affected by defects due to lattice relaxation of the epilayer.