The Urban Heat Island effect on slope stability was investigated using experiments, field observation and numerical simulation. A series of laboratory tests shows that the shear strength of Xiashu clay soil (Nanjing, China) is influenced by temperature, water content and density. The numerical model indicated the average temperatures of the potential failure zone in the modeled slopes increased by about 3.8 °C during a 90 day period from 13th June; the average difference between the urban and rural environments was 2.46 °C. The temperature fields were combined with the cohesion–temperature relationship to calculate the safety factor of the modeled slopes using the Sweden Slice Method. The results indicate that the safety factor of the slopes reduces during the hot summer weather, with the effect most pronounced in the urban environment. The conclusion from this preliminary study is that the Urban Heat Island has a negative effect on the slope stability in this region.