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There has been no study on the impacts of street tree interval on the street-level radiant environment, although it has been a key factor in the design of thermally comfortable streets. In this study, the variation in pedestrian mean radiant temperature (MRT) is investigated as a function of the tree interval using a newly developed multilayer MRT model. Tree size and street size are considered in the model experiments. The results show that MRT was most effectively reduced by large trees, indicating that they may be planted at wider intervals compared to smaller trees to achieve comparable MRT reductions. Furthermore, as the tree interval decreases, MRT reduction was increased exponentially by small trees, while MRT reduction was increased linearly by large trees. Therefore, urban planners can reduce pedestrian exposure to radiation by planting either larger trees or closely spaced smaller trees. These results provide insight into optimal configurations of street trees for maximum reduction of MRT in a variety of urban canyon configurations.