Outdoor thermal comfort has been a widely concerned issue in tropical and sub-tropical cities. In order to assess the conditions of outdoor thermal comfort, quantitative information on different spatial and temporal scales is required. The study in this chapter employs a numerical model to examine the spatial and temporal variations of mean radiant temperature (Tmrt), as an indicator of radiant heat load and outdoor heat stress in high-density sub-tropical urban environment in summer. The SOLWEIG model is found to simulate the six-directional shortwave and longwave radiation fluxes as well as Tmrt very well. Simulation results show that urban geometry plays an important role in intra-urban differences in summer daytime Tmrt. Open areas are generally warmer than surrounding narrow street canyons. Street canyons are sheltered from incoming direct solar radiation by shading of buildings, while open areas are exposed to intense solar radiation, especially along the sunlit walls where high Tmrt is observed due to reflected shortwave radiation and longwave radiation emitted from the sunlit building walls. The present study confirms that there are great potential in using urban geometry to mitigate high radiant heat load and daytime heat stress in the compacted urban environment. In high-density sub-tropical cities where high daytime Tmrt causes severe thermal discomfort in summer, dense urban structures are able to mitigate the extremely high Tmrt and improve outdoor thermal comfort. However, the shading strategy has to be cautious about air ventilation in such a dense urban environment.