Abstarct: Flood occurrence is one of the natural destructive phenomena that every year it leads to financial losses and great physical harms around the world and has been the concerns of different scientists and researchers which reduce the damages caused by flood; therefore، the various hydrological models have been formed to simulate rainfall-runoff in recent decades، which flood volume، peak flow and time to reach the peak flow could be calculated by means of these models. In the present study WMS/HEC1 model was used to simulate the rainfall-runoff in Kasilian basin. Kasilian basin is located in Mazandaran province in northern Iran. In this study، by using land use maps and soil type map، CN curve number was calculated and it was used in the simulation. The Rainfall-runoff simulation once was conducted in Valikbon sub-basin in an area of 67.18 km2 with three rainfalls Apr. 02، 2007، Apr. 03، 2009 and Mar. 30، 2012 that Apr. 03، 2009 rainfall was used to calibrate and other two rainfalls were used for the authenticity survey. Then the total Kasilian basin to Shirgah which is 342.78 km2 was simulated using the rainfalls dated Apr. 01، 2008، Apr. 03، 2009 and Sep. 15، 2012. Initially، the SCS unit hydrograph method was performed with three input variables of CN، Rtimp and STRTL. The output of calculation and observation were closed together، but to calibrate and for more precise، the Clark unit hydrograph method was also used and the error was decreased dramatically. In the rainfall-runoff simulation in shirgah sub-basin، the error rate of flood volume، peak flow and time reach the peak was 8/8, 3 and 0 percent Respectively and the error rate for the above three parameters in the whole Kasilian basin was 8/4 percent Therefore، it was concluded، the rainfall-runoff modeling using the WMS/HEC1 model is a good option to simulate in the Kasilian basin and this model can be used in future studies.