Based on traditional reverberation theories such as Sabine's or Eyring's theories, the reverberation energy decay becomes linear in logarithmic scale of energy. However, this is valid only in diffuse sound fields. Sound fields in actual rooms are non-diffuse sound field. It has been well known that nonlinear reverberation decay occurs in a rectangular room with uneven distribution of absorption. Hence, reverberation times measured in such rooms are not correspondent with those calculated by the theories. Reverberation theories for such rectangular rooms have been proposed by some researchers. However, these theories have not been sufficiently validated. In this study, a new mathematical model of reverberation decay in rectangular rooms with uneven distributions of absorption is proposed. Most of previous theories contained only exponential decays. The proposed model in this study contains not only exponential decays, but also power law decays.