An experimental and numerical study on the three-dimensional natural convection-radiation conjugate heat transfer in the enclosure with heat generating chip has been performed. A 3-dimensional simulation model is developed by considering heat transfer phenomena by conduction-convection and radiation. Radiative transfer was analyzed with the discrete ordinates method. Experiments are conducted in order to validate the numerical model. Comparisons with the experimental data show that good agreement is obtained when the radiation effect is considered. The effects of the thermal conductivity of the substrate and power level on heat transfer are investigated. It is shown that radiation is the dominant heat transfer mode and the conductivity of the substrate has important effects on the heat transfer in the enclosure.