A sub-channel analysis code is commonly used for evaluating thermal-hydraulic behaviors in a fuel assembly. Since the spatial resolution of the sub-channel analysis code is “sub-channel” level, the sub-channel analysis code can provide more detailed thermal-hydraulic information than a conventional one-dimensional nuclear reactor safety analysis code, whose spatial resolution is “reactor core” level. To predict the thermal-hydraulic behavior, it is essential to predict the distribution parameter, which significantly affects the void fraction prediction. However, existing sub-channel analysis codes assume that the distribution parameter is unity for each sub-channel or the distribution parameter obtained for the “reactor core” level is adopted. The current study developed the constitutive equation of the distribution parameter at the “sub-channel” level, based on the methodology proposed by Julia et al. (2009). The developed constitutive equation of the distribution parameter was successfully validated against the NUPEC steam-water void fraction data collected in an 8×8 rod bundle under prototypic BWR conditions.