Measurements of change in length and change in lattice parameter were conducted at identical temperatures on CoCrNi in the temperature range 300 to 1273K. Length changes, ΔL, were measured on an unconstrained horizontal extensometer. Lattice parameter changes, Δa, were observed using an insitu-neutron diffraction technique. The measurements can give us the relative expansions ΔL/L and Δa/a. At higher temperatures additional atomic sites were found to be generated and the difference between the two expansions could be represented by 3 (ΔL/L-Δa/a) =exp (-Q/RT). The observed activation energy would lead us to deduce the concentrations of monovacancies, divacancies and trivacancies. Furthermore, we can also predict the formation entropy. The thermally generated point defect research of high-entropy alloy would help us to get more physical properties information at high temperature.