This dissertation discusses the determination of urban heat islands based on examples of smaller Slovenian cities together with the effect of green areas on the urban heat islands. With the overview of municipal spatial planning documents we have gained insight how much municipals are protecting green areas and how do they plan to reduce the phenomena and effects of urban heat islands. In the practical part of this dissertation we used satellite imagery with different spatial resolutions (MODIS and Landsat 8). The results of the practical part have shown, that for determination of urban heat islands in smaller cities more accurate satellite data is required, which in our case is the data of the satellite Landsat 8. We proved that the effect of urban heat island is also present in smaller cities and that green areas affect the magnitude or the size of the urban heat island. Municipals could use these results for planning greener cities and directing the population, to reduce the size and effect of the urban heat island. This subject is not yet discussed in municipal spatial planning documents and green strategies of the cities. On the contrary, protection of green areas is included in the documents, yet in reality, there is no proper supervision on the field.