This study aims to investigate the importance of local thermal factors contributing to urban heat island (UHI) for the cities of Changchun, Yanji, and Linjiang in Northeast China over the past 50 years. Changchun is a large city, the capital of Jilin Province, and Yanji is a middle one, while Linjiang which locates in a lap of hills, is a small town situate in the down of heavy industry centre of Northeast China. Statistics shows that the rank order of their urbanization rates (URT) is the same as the city sizes. The observation shows that their minimum temperature trends (MNTT) are much greater than their maximum counterparts (noted as MXTT), and the winter trends are greater than their summer counterparts. The rank order of the MNTT is contrast to the MXTT and approximately the same as the URT, while the difference of MNTT and MXTT is proportional to the URT. The land use, anthropogenic heating, and air pollution are three major factors for UHI, and the later two are proportional to the URT. Inter-comparison reveals that the air pollution and anthropogenic heating are key factors for the UHI, because their combination could explain the major characteristics of the trends and orders, while the land use doesn’t. The results also manifest the importance of local geographical environment in Linjiang, which lead to a weak wind climate that has the thermal factors localized. The localization may be one of the causes that produced the strongest MNTT and MXTT in Lingjiang in wintertime. Nevertheless, the difference of the MNTT and MXTT can reduce, to some extent, the influence of the geographic factors on the temperature trends because it is proportional to the URT. Hence, the difference can be regarded as a good climate index in representing urbanization. Besides, the results also imply that the important impact of air pollution (aerosols) on long-wave radiation should be considered not only in observation, but also in numerical model of atmosphere. A suggestion is to increase green land, and meanwhile to reduce air pollution and anthropogenic heating for mitigating city climate warming.