An automatic power grid synthesis method and a computer readable recording medium for storing a program thereof for synthesizing power grid in a circuit area are provided. The circuit area has at least one power consuming module therein and at least one power pin disposed around the circuit area. The method includes the following steps. First, select at least one representative point in each power consuming module. The circuit area is divided into a plurality of regions according to the positions of the representative point(s) and the power pin. An overall power grid density of each region is calculated in accordance with the position of each power pin and the power consumption or current requirement at each representative point. Finally, the power grid synthesis is performed in each region according to the corresponding overall power grid density of the region.