Urban green infrastructure can to a certain extent mitigate urban warming. However, the cooling effect of plants varies with space, time and plant-specific properties. To contribute to our understanding of the cooling effect of vegetation on urban surface and air temperature, 21 parks in Addis Ababa were studied. Air temperature and humidity were measured for 60 plots in nine of the parks for 15 days. Furthermore, the thermal band of Landsat ETM+ was used to examine the cooling impact of all 21 parks on a larger spatial scale. Linear mixed-effects models were used to examine the relationship between characteristics of the vegetation and observed temperature. It emerged that Eucalyptus sp. had a significantly higher cooling effect than any other species group (P < 0.05) and the species with the least effect on temperature were Grevillea and Cupressus. On a larger spatial scale, the cooling effect of parks on their surroundings (Park Cooling Intensity, PCI) was positively related to the NDVI and area of parks (P < 0.01). A negative relationship was observed between PCI and park shape index (SI). The range within which the cooling effect could be observed (Park Cooling Distance, PCD) was positively related to SI and park area. The maximum PCI was 6.72 °C and the maximum PCD was estimated at 240 m. We conclude that the cooling effect is mainly determined by species group, canopy cover, size and shape of parks. Thus, the study provides insights regarding the importance of species choice and spatial design of green spaces in cooling the environment.