This study investigated impacts of urban forms on the future thermal environment over Beijing, the capital city of China. Beijing is experiencing remarkable urban expansion and is planned to undergo the transformation of urban forms from single-centric (compact-city) to poly-centric city (dispersed-city). Impacts of urban forms on the future thermal environment were compared and evaluated by conducting numerical experiments based on a regional atmospheric model coupled with a single-layer urban canopy model as well as future climate forcing output from a global climate model. Results show that a dispersed city is efficient in reducing mean urban heat island intensity, but produces larger thermal loading and deeper thermal feedback at the regional scale compared to a compact city. Thermal comfort over downtown areas is reduced in compact-city scenario under future climate conditions. Future climate contributes almost 80% of the additional thermal loading over urban areas, with the remaining 20% contributed by urbanization (for both the compact-city and dispersed-city scenarios). The thermal contrast between the two urban forms is dominated by the expected future climate change. This study leads to two complementary conclusions: (i) for developing assessments related to current climate comfort, urban form of the city is important; (ii) for assessing future climate change impacts, the areal coverage of the city and urbanization extent emerges to be more important than the details related to how the urbanization will evolve.