Thermal stress caused by temperature distribution in pavement slab is important in the structural design of airport concrete pavements. Airport concrete pavement slab is thick; therefore, temperature in the slab varies nonlinearly throughout its depth. Thermal stress caused by nonlinear temperature distribution differs from that of linear temperature distribution. In this study, thermal stress caused by nonlinear temperature distribution was calculated by using temperatures and restraint strains measured in an experimental concrete pavement with 460-mm-thick slabs. The calculated thermal stress differed from curling stress because of the linear temperature distribution. Temperature distributions in concrete slabs of various thicknesses were computed by solving the heat transfer equation with the control volume method. This calculation revealed that temperature distributions in thick concrete slabs were highly nonlinear. Thermal stress was analyzed to compute the nonlinear thermal stress distributions in concrete slabs from the computed temperature distributions by using a three-dimensional finite element model. On the basis of the analysis, effects of slab thickness and nonlinear temperature distribution on thermal stress were discussed. The effect of nonlinearity was found to be pronounced at a slab thickness greater than 300 mm. In this range, the reduction factor for taking into account the effect of nonlinear distribution was larger than 0.7. It was also found that thermal stresses in summer and spring were relatively large.