Disclosed are an apparatus for measuring an in-situ crosslink density includes a support configured to fix or support a cross-linkable structure, a light source configured to irradiate light for crosslinking to the cross-linkable structure, and a probe configured to provide in-situ micro-deformation to the cross-linkable structure, wherein the in-situ crosslink density of the cross-linkable structure is measured from a stress-strain phase lag of the cross-linkable structure by the in-situ micro-deformation, a method of measuring the in-situ crosslink density, a method of manufacturing a crosslinked product, a crosslinked product obtained by the method, and a polymer substrate and an electronic device including the crosslinked product.