A new concept is proposed for a high conversion light water reactor (HCL WR) that achieves both high conversion and high burnup while maintaining a negative void reactivity coefficient. This HCLWR has a flat “pancake” core with thick axial blankets. By using the flat core, a potential problem of HCLWRs, the positive void reactivity coefficient, can be reduced by neutron leakage, and a fuel assembly of very tight lattice pitch can be used. The leakage neutrons are utilized in the axial blankets to enhance the conversion ratio. With the axial blankets, the core shows a small value for the axial power peaking factor, and the plutonium enrichment can be largely reduced by the neutron reflection, including the fast fission due to 238U, compared with the bare core. Moreover, upgraded burnup characteristics can be obtained by the accumulation of fissile plutonium in the blankets. The flat core with the blankets can be applied to a small- or intermediate-scale light water reactor. The analysis combines a cell burnup calculation and a one-dimensional burnup calculation based on the diffusion method. The evaluation method of the void coefficient is validated for the flat core with axial blankets under practical, controlled conditions. Moreover, the accuracy of the present method is validated for the transport effect on the major physics parameters of interest by using the one-dimensional transport code ANISN and the Monte Carlo code VIM.