Trees have been found to have significant cooling effect on outdoor thermal environment. This study explores the characteristics of a tree component (crown) that accounts for a large portion of the cooling effect. A field measurement was conducted in the Gold Coast, Australia, which has a subtropical climate (Cfa), according to the Köppen climate classification. The characteristics of the tree crown were indexed by its crown diameter, crown height, sky view factor (SVF), and leaf area index (LAI), based on which four new indicators that describe crown characteristics were proposed: gross canopy index (GCI), leaf layer index (LLI), crown volume index (CVI), and total canopy index (TCI). The former two are basic indicators and the latter two are sophisticated indicators. The results showed that the two sophisticated indicators were significantly associated with the mean radiant temperature beneath the tree crown and that increasing the value of each indicator by 10% could help reduce the mean radiant temperature by 0.67 °C (CVI) and 0.83 °C (TCI) respectively, if other predictors were fixed. Similarly, increasing the value of each basic indicator by 10% could help reduce the mean radiant temperature by 0.78 °C (GCI) and 0.066 °C (LLI) respectively, but their cooling effects were less significant than CVI and TCI. The thermal effect of the two sophisticated indicators was more significant than that of the two basic ones. Among all, TCI was found to be the best predictor of the outdoor thermal environment underneath the tree crown. This study proposes new effective indicators to quantify the cooling effects of trees and provides a rationale for plant selection in tropical/subtropical climates to achieve a more comfortable outdoor thermal environment.