The climate of the planet has been changing ever since. One of the factors that affect the climate of the planet is water vapor. In this study, the data of 27 synoptic stations of northwest Iran were used to estimate the relative humidity trend in the northwest of Iran during the statistical period of 25 years. MATLAB and software with dimensions of 27×300 were obtained from the data representing time (moon) and columns representing the location. Using the Kriging interpolation method in the geographical information system, the trend maps and the monthly trend slope for January 1989 to December 2013 were developped. In this study, a regression model was used to calculate the time series process using least squares method, and the non-parametric Mann-Kendall test was used to identify the process. The results showed that the highest range of negative trend areas was observed in March with 81.5% of the area in the region, from -0.3 to -7.7% in decades, the highest in Bijar and its minimum in Zarinehabatu. Also, the lowest range of negative trend areas in October is seen with 3.7% of the area in Takab, a decrease of - 4.0% in decades. The largest number of regions with a positive trend in September and November with 14.8% of the area in Mianeh, Zanjan, Khoramdarah, Hamedan Nogheh, Hamedan, Kangavar increased 0.3-0.5% in September and 2 in decades. 0 to 0.4 percent increase over the decade in November, and the lowest range of positive trends in the months of August and February with 3.7 percent of the area in Parsabad and Kangavar was 0.3 and 0.2 percent respectively, in the decade is visible.