Riga city (56°57'00'' N and 24°06'45'' E) is located close to the Gulf of Riga, Baltic Sea coast and having population approximately 800 000, is the largest city in Latvia. The weather conditions are affected by its presence in moderate climate zone. Long-term changes of the urban heat island in Riga were investigated. The time series of annual and monthly mean, average maximum and minimum temperature differences between Riga-University as urban and two other different types of stations: Riga (suburban) and Jelgava (rural) for the period 1941-1999 was analysed using linear regression and Mann-Kendall trend test. The significant positive trend of annual temperature differences for the urban-rural (0.0044 °C yr -¹) and suburban-rural (0.0087 °C yr -¹) pairs of stations, and negative trend for urban-suburban (-0.0049 °C yr -¹) pair have been found. Differences for urban-rural mean temperatures increase in summer, for suburban-rural - in all seasons, especially from May to September. In terms of differences for urban-suburban mean temperature, it shows the significant negative trend, except summer, when changes of mean temperature differences are not observed. There is also significant negative trend of minimum temperature differences between urban-suburban pair for all months. Differences of mean monthly maximum temperature show the significant positive trend only for winter season. In summary the following conclusions reached from the analyse: (i) There are seasonal differences of changes in the intensity of urban heat island in Riga - it increasing during the warm season, but no changing during cold season. (ii) Transition of the land use from predominantly rural to more urban setting have an impact on the trend of temperature and on the spreading of urban heat island phenomena on the surroundings.