This paper presents twelve full-scale experiments conducted on radiation from compartment fires to a target wall. The impact of different window sizes, separation distances between the fire building and a target wall, and different fuels on radiation heat fluxes on target wall is investigated. The contribution of external flame to radiation heat flux on target wall was studied. The emissivity of external flame is analyzed which shows that the experimental value is larger than that suggested by previous studies. The experimental data have been compared with theoretical results. The discrepancy between experimental and theoretical data has been explained. The limitations and recommendations of some equations have been discussed. Furthermore, a modeling of radiation heat flux on target wall from post-flashover compartment fire is proposed. This work is expected to be used as the basis to develop modeling of fire spread between buildings, modeling of post-earthquake fire spread and modeling of urban/wildland interface fire spread.