Coefficient of thermal expansion (CTE) for thin film has been measured only from change in thickness because thin film has to be constrained on a solid substrate. However, thin film CTE shows different values depending on the supporting solid substrate. Here, a novel measurement method is suggested to quantitatively measure the in-plane thermal expansion of thin films floating on a water surface. In-plane thermal expansion of thin films on water surface is achieved by heating the water. The CTE is measured through a digital image correlation (DIC) technique. The DIC tracks displacement marks deposited on the film surface, and the in-plane thermal strain is defined as the change in distance between the patterns. The method can be applied to measure the CTE of polymer, metal, and graphene with a thickness ranging from a micrometer to one-atom-thickness. The in-plane thermal expansion of the polystyrene (PS) thin film decreased as the film thickness decreased. The negative CTE of graphene is also successfully explored without any substrate effects or complicated calculations. The CTE measurement method can provide understanding of the intrinsic thermal expansion behavior of thin films including emerging two-dimensional materials.