Abstract: It is theoretically demonstrated that the influences of terrain height and orographic heterogeneity are important on the flux of surface long wave radiation over a large scale grid area. The results of numerical experiments show that mean terrain height and orographic relative standard deviation over a large scale grid area are two important factors which have influence on the mean flux of surface long wave radiation. Over a complicated topography region, because of the nonlinear increment of the relative standard deviation of topographic elevation with mean terrain height, the mean value of surface long wave radiation flux has a nonlinear increment by the dual-effects. But, the influences of orographic heterogeneity on surface long wave radiation are larger in comparison with the heterogeneity of surface temperature although the latter also has an evident impact on surface long wave radiation flux. It was proved under the homogeneous supposition that there are three parts of the relative errors in calculating the effective radiation flux over a complex orographic region i.e. (1) the deviation of topographic heterogeneity from terrain height; (2) the deviation from surface temperature heterogeneity; (3) the mixed deviations produced by orographic heterogeneity and surface temperature heterogeneity. The first error is larger, which sometimes reaches 40% in the mean effective radiation flux calculated over a heterogeneous topography region. On the other hand, the second error, which is the mixed deviations between the terrain height heterogeneity and surface temperature heterogeneity, is much smaller. Thus, when the effective radiation flux is calculated over a large scale grid area which has a complex topography, the influences of orographic heterogeneity should be considered in the climate model, and the surface temperature heterogeneity effects may be omitted.