Many researches around the world have indicated the gradual increase in mean annual temperature. Yet there are some reports on reduction of this parameter. In this research we investigated the long time trend of mean annual temperature in 34 synoptic stations of Iran (2 stations in Cool humid climate, 14 stations in Temperate humid climate, 11 stations in Steppe climate, and 7 stations in Desert climate-based on the Koppen climatic division) with a minimum record of 30 years by t-test. Result showed that for the whole time horizon of data, there was a positive trend in 50% of stations, while 41% of stations had negative trend. Considering significance level (=5%), there were 3 zones of positive, negative, and without trend for mean annual temperature in Iran. However, it was hard to define a specific spatial scheme for such a division. The results were supported by Man-Kendall method, while low harmony was found with Wald-Wolfowitz. As far as record length is concerned, under a common time period (1968-1998), 65% of stations showed a positive trend, while 32% of them followed a negative trend. There were some shifts for one trend direction to another one for some of the stations of the study, yet with no well-defined spatial structure. In this case and at 5% level of significance, 44, 15, and 41 percentages of the stations possessed positive, negative, and without trend for the parameter under study, respectively. In general the behavior of trend direction was different for different climates and no specific pattern was found. By the result, one may hypothesize that, in future, more regions will experience higher temperature values. Some stations did not show significant trends, yet their positive trends may be a clue for future warming. Including the four recent years of 1999-2002 to the data, verified the results of trend analysis. All of the stations showed higher average mean annual temperature, as was compared with average of corresponding to the common period of 1968-98.

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