This paper introduces and discusses the basic concepts and structure of IHACRES model based on unit hydrograph principles. As a lumped parameter conceptual rainfall-runoff model, IHACRES model has a simple structure, clear physical explanation of the dynamic response in the watersheds and a parsimonious number of parameters. IHACRES model has been widely studied abroad and it is considered as a successful model. Four watersheds were seleceted as case study with each watershed being assumed gauged basin. IHACRES model and Xinanjiang model were calibrated separately, then they were used to simulate the streamflow in the other ' ungauged' watersheds. The result shows that IHACRES model is better than Xinanjiang model in Runoff Simulation in ungauged basins.