Abstract: Canopy transparency of garden trees has an important influence on the shading function and other ecological benefits. Ten common street trees in Zhaoqing City were selected, and the canopy photos were taken by digital camera. Canopy transparency was measured by Photoshop image processing method to verify the correlation between tree parameters, canopy transparency, shade area and shade illumination. The shading function was comprehensively evaluated by principal component analysis. The results showed that: (1) Delonix regia, Alstonia scholaris, Terminalia neotaliala and Ficus virens had the highest transparency, while Delonix regia, Alstonia scholaris, Terminalia neotaliala and Mangifera indica had the highest shading illumination. (2) Canopy transparency (CT), height under branches (UBH), tree height (TH), and diameter at breast height (DBH) were positively correlated with the shading illumination (SI). Canopy width (CW), canopy height (CH), DBH, TH were positively correlated with shade area (S). UBH, DBH, TH were positively correlated with CT. SI was significantly negatively correlated with CH, and layer transparency was significantly positively correlated with SI. Canopy width was the dominant factor to determine the shade area, and canopy transparency and canopy height were important factors to determine the shading illumination of trees. (3) The top three trees with the comprehensive shading function were Ficus microcarpa, Ficus virens and Chukrasia tabularis. The measurement of canopy transparency provides a new idea for the study of shading function of garden trees.