Nowadays we witness some increasing trend of natural resources degradation and human interference in natural cycles is manifested in destroying plants in watershed regions, illegal land use, developing impervious surfaces and affecting hydrological regime of the basin. In recent years improper and unplanned development of residential areas and consequences degraded forest lands led to especially in mountainous regions increasing changing of rain into runoffs and also decrease the time of concentration, lag time and time to reach peak flow. the foregoning cases is true for Khoramabad watershed. Therefore it is necessary to investigate the effect of land use changes especially development of residential and urban areas on the runoff rate. Having knowledge about the increased runoff rate and its behavior requires some detailed statistics about hydrological condition or river basin discharge data. this goal can not be realized in rivers with seasonal or intermittent flow so the application of hydrologic models for runoff simulation and hence application of geographical information system ( GIS ) for visualization and analyzing site data are adopted in numerous studies. In this research having been mapping land use, CN and runoff Coefficient (C) in years 1985, 2000 and 2013 were provided and their variations in the studied period were analyzed. and it was found the most important land use changes included degration of forest areas, increased urbanization and buildings and in some sub basin, reduced agricultural area among many others. In sub-basin 4, the highest changes were attributed to urbanization and rural development and in this sub-basin, from 1985 to 2013 CN value varied from 82.135 to 85.28 and at the same period C value ranged from 0.51 to 0.57. in order to determination of the impact of land use changes, watershed behavior was simulated based on precipitation and recorded hydrographs. Transformation of precipitation to runoff was done using curve number model (CN) in HEC HMS software. After modeling, calibration and validation, resulted model was applied for watershed hydrological behavior simulation and it was run for years 1985, 2000 and return periods 2 to 100 years. The results showed that land use change especially increased urban and rural areas development and consequently increased C caused an increase in Q peak from 42.5 to 400 percentage for 1985-2000, from 61.25 to 700 percentage for 1985- 2013 and from 13.15 to 33.33 percentage for 2000-2013. And also the time of concentration and time to peak decreased to -12/45 and -9/64, respectively. Also it was revealed that as return period of precipitation was increased, discharge variations percentage was decreased since 1985 to 2013 that indicating low impact of land use changes on longer return period’s discharges .so that, the peak flow of 2 and 100 return periods from 1985 to 2013 increased to 22.558 and 16.874 percentage, respectively . Key words : land use change , urban and rural area development , HEC HMS Model, khoram abad watershed , peak flow.