NiC/Ti neutron supermirror plays a key role in improving the capability of neutron optics. In order to improve the performance of the neutron supermirror, a study in microstructures and interfaces of NiC/Ti multilayers with varied NiC layer thickness was presented. The thickness, interface roughness and crystalline state of NiC/Ti multilayers were represent by Grazing incidence X-ray reflectivity and X-ray diffraction, respectively. The study results indicate that the NiC-on-Ti interface roughness was almost constant except the thin NiC layer, which has the thickness less than 2.5 nm, however the Ti-on-NiC interface roughness has relatively large changes. The NiC layers transferred from amorphous to polycrystal, as the NiC layer thickness increase from 2.5 nm to 5.5 nm, and both two interfaces and the size of Ni (111) crystalline grains almost kept constant as the NiC layer thickness continue to increase. This different variation between Ti-on-NiC and NiC-on-Ti interface roughness can be attributed to the microstructures changes resulting from varying NiC layer thickness.