The analytic models of warmest slope(WS) facing south or north calculated according to the solar radiation received during any given day are proposed in this study. North hemisphere is taken as an example.1. In the half year of winter,WS exists for south slope,and increases with latitude.2. In the half year of summer, there are two threshold latitudes: ψ₀₁(≤43. 3°N)and ψ₀₂(≥63. 5°N): when ψ＞ψ₀₁, WS exists for south slope,when ψ＜ψ₀₁orψ＞ψ₀₂,WS exists for north slope.3. The daily solar radiation on the slope with WS decreases with the latitude in the half year of winter. In the half year of summer ,it increases with latitude on the north slope when ψ＜ψ₀₁. It is equal to the value of horizontal surface at ψ₀₁ on south slope when ψ＞ψ₀₁.