In order to provide food to more than six billion people, global irrigated croplands have expanded dramatically in recent decades. The role of irrigation in modifying the regional climate has been widely recognized in recent studies. India, one of the most intensely irrigated regions of the world, was selected as the simulation experiment region to determine the impact of irrigation on regional climate. Two 11-year (January, 1990-December, 2000) simulation experiments were conducted by using the Regional Integrated Environmental Model System (RIEMS) version 2 over Indian region: (i) Rainfed cropland (RFC) and (ii) Irrigated cropland (IGC). We discarded the first year from the January 1990-December 1990 model running as equilibration time, and reported results of the final 10 years (January 1991-December 2000) as differences between the two cases (IGC-RFC). RIEMS2.0 uses the Biosphere-Atmosphere Transfer Scheme (BATS 1e) as its land surface process scheme. In BATES 1e root zone soil moisture is set to field capacity throughout the year in the irrigated crop area, and the soil moisture is the function of rainfall, evapotranspiration, and soil feature in other non-irrigated areas. Over the 10 year time period, the temporal difference of the two regional climate model sensitivity experiments showed that a regional irrigation cooling effect exists with annual averaged 2 m air temperature decreasing 1.4℃ and the precipitation rate increasing 0.35mm/d at the national scale. From the spatial difference of temperature and precipitation we found that the irrigation effect on climate was not only confined to the near-surface atmosphere in irrigated grid cells, but also spread to adjacent grid cells. The irrigation cooling effect can contribute to the increased latent heat flux and decreased sensible heat flux. The increased precipitation rate depends on the offset between the positive convective rainfall and the negative large scale none-convective rainfall. The positive convective rainfall is intrigued by the two factors that can fuel deep convection, one is added water vapor and the other is a greater latent heat fluxes that result from intensified evapotranspiration because of irrigation. The large scale none-convective rainfall decreased because of the weak divergence circulation of wind field at 850hPa which weakened the water vapor transportation from the sea to India peninsula. The results of seasonal difference indicated that the climate of pre-monsoon season and June is more sensitive than monsoon season (July to September) to irrigation. The national averaged change in temperature was 3.18℃ in pre-monsoon season and 0.43℃ in monsoon season, respectively. This seasonal differences can be explained by the fact that pre-monsoon season is much drier than monsoon season. During the dry season evapotranspiration difference between irrigated cropland and rainfed cropland is greater than that of the wet season. The results of this paper are limited by the BATS treatment that sets the soil moisture as a constant to field capacity throughout the year without considering about the seasonal variation of irrigation. A much more reasonable irrigation parameterization scheme is supposed to be coupled to BATS1E.