Abstarct: The study of discharge variations of Qanats is very important for water resources management and planning. In this study, considering the importance of the subject, in order to investigate the trend changes of discharge of the Qanatsin the Shahrood and Miami regions, their trend was investigated in three monthly, seasonal and annual scales using the Trend-Free Pre-Whitening Mann-Kendall (TFPW-MK) method. The data used include the data of the 9 selected Qanats during the 23 years’ period (1966-1999) in the study area. In this research, the linear trend slope was estimated in the sample data using the Theil-Sen approach (TSA). Then, using the TFPW method, the correlation coefficient of the data was eliminated and the time series of the discharge pre-whited. Then, the trend changes of discharge in the original time series and pre-white series were investigated using the Mann-Kendal test (MK). The results of this study showed that in most stations (8 Qantas from 9 Qantas), the discharge trend is mainly descending, and in the last 24 years the decrease in Qanat discharge is quite tangible.