The effects of heat load, ablator density, and backup structure, etc. upon the heat shield performance of the lightweight phenolic carbon ablators named LATS were investigated using a one-dimensional ablation analysis code. The ablator density was assumed to be from about 260 to 1000 kg/m³. Heat flux time histories of a rectangular pattern were assumed, where cases of constant heating duration time and constant accumulated heat load (up to 600 MJ/m²) were considered. The heating level was assumed to be from 1 to 10 MW/m², which means that the ablator surface is in the region of diffusion control oxidation/sublimation. The materials of the backup wall are assumed to be aluminum, stainless steel and high density CFRP. Main findings are: (1) For a low heat flux q with the same heating duration time t_q, the necessary thickness, with which the maximum back surface temperature equals to the pre-determined allowable temperature, is nearly constant as the density ρ_v changes. On the other hand, the necessary thickness increases largely when q is larger and ρ_v is smaller. The ablator necessary mass increases with the increase of ρ_v and q for the same t_q. (2) When a backup wall is attached, the necessary thickness decreases and the necessary mass including the wall mass increases. (3) For a constant accumulated heat load, necessary thickness and mass decrease for a higher heat flux q especially when ρ_v is high. (4) A lower density ablator with a CFRP backup wall gives the lightest mass of the heat shield system for most of the parameter range among the three wall materials. (5) For a high heat flux, selection of a lower density ablator gives a larger necessary thickness.