Abstract: The impacts of surface heating heterogeneity on the applicability of similarity theory in the unstable surface layer are investigated using idealized Large-Eddy Simulation (LES) experiments with prescribed heterogeneous surface heat flux forcing. It is found that the mean profiles in the boundary layer are hardly affected by surface heterogeneity. However, heterogeneous surface heating on a relatively large scale can induce organized large-scale secondary circulations. In this case, the fluxes between COLD and WARM patches are distinct up to the upper boundary layer. On the other hand, there is no such organized secondary circulation developed in smaller heterogeneous scales, and the fluxes are mixed quickly in the low boundary layer. For all heterogeneous cases (from small to large heterogeneous scales), the inter-patch advection greatly impacts the surface layer structure of patches. Therefore, the flux-gradient relationship in those surface layers deviates from the similarity theory, with the deviation more for momentum than for heat. Finally, an improvement to the Mosaic approach based on the similarity theory and widely used for estimating subgrid heterogeneous surface fluxes in current atmosphere models is proposed.