For the virtual reality (VR) applications, it is important to quantify the just-noticeable-difference (JND) profile of the image such that immersive experience could be provided. In this paper, we first propose a test plan of subjective quality assessment for VR image, then a foveated JND (FJND) model that is capable of modeling the basic vision properties to quantify the correlation between eccentricity and conventional JND. The psychophysical experiments discover the relationship among the relevant factors and quantify the foveated JND that could be used for immersive VR applications.