In order to facilitate discussions based on quantitative analysis about the end effect, which was often talked about in connection to burnup credit in criticality safety evaluation of spent fuel, a burnup importance function was introduced. This function showed the burnup effect on the reactivity as a function of the fuel position; an explicit expression of this function was derived by considering a change in reactivity with respect to a slight variation in fuel burnup. The burnup importance function was applied to the Phase IIA benchmark model that was adopted by the Organization for Economic Cooperation and Development/Nuclear Energy Agency Expert Group on Burnup Credit Criticality Safety. The function clearly displayed that burnup importance of the end regions increased (a) as burnup, (b) as cooling time, (c) in consideration of burnup profile, and (d) in consideration of fission products. Comparison of the burnup importance for different initial enrichments was also shown.