Results of assessment of the future climate change impacts is associated with some uncertainties. Considering the range of uncertainties increases reliability of the results. In this study, climate change impacts on daily precipitation, maximum and minimum temperature of Sira basin are assessed using LARS-WG model, for 2036-65 period. Accordingly, uncertainty of new emissions scenarios (RCP2.6، RCP4.5، RCP6 و RCP8.5) of HadGEM2 and climatic variability are investigated. Climate variability is a major source of uncertainty which is ignored in most previous studies. Based on the results, for precipitation, the range of the future scenarios have considerable overlaps with the historical ranges. So, it cannot certainly be concluded that whether precipitation increase or decrease in the future. But in the mean of the possible scenarios, it is expected that annual precipitation will change between -9% to +1% in the future. About temperature, the range of the future temperature scenarios have no overlaps with the corresponding historical ranges, and there is a grate certainty for temperature increasing in most of the months, and change in the seasonal cycle of temperature. In the mean of the possible scenarios, it is expected that average temperature will rise between 2.1 to 3 ºC in the future.