Urban growth can profoundly change the urban landscape structure, ecosystem processes, and local climates. Timely and accurate information on the status and trends of urban ecosystems is critical factor for sustainable development strategies and it improves the urban residential environment and living quality. In recent years, the migration from rural area to urban is major driver for expansion of Ulaanbaatar and city was urbanized a quite rapidly in the past twenty years. The aggregation of high temperature occurs particularly in the more constructed area as much as evidenced in the city center, meanwhile the reduction and fragmentation of low temperature is even more apparent in suburban areas of city. Urban Heat Island (UHI) is the most significant issue towards urbanization and sustainable development of urban around in the world. This study analyzed the Land Surface Temperature (LST) differences on time series (1995 - 2016) of Ulaanbaatar, Mongolia and the relationships between LST and Normalized Difference Vegetation Index (NDVI) (green spaces in the city) using statistical analysis, such as box-plotting and regression. The analysis basically focused on the relationship between LST and change of green space area influenced by human activities. The analytical methodologies used in this study could help to calculate the urban thermal environmental functions under conditions of urban expansion and explore the local climate change of this city. In addition, the satellite images data based on Landsat time series between 1995 and 2016 were analyzed for change detection mapping. Landsat series images were gathered by thermal mapper (TM), enhanced thermal mapper plus (ETM+) and operational land imager (OLI). The results show that relationships between each impact become an important determinant of changes in the urban thermal environment. Furthermore, seasonal factor significantly impacts to the strength of this relationship and it is being to the highest contribution indices values in summer.