In order to understand the course of a river in human life as water supplier which causes destruction and damage, it is necessary to determine the flood zone with great accuracy. The aim of the present study is determining the flood zone of Khesht river in Khesht and Komarej division in Kazeroon city, situated in south western of Fars with the length of 10 Km. In this research on of the newest type of aerial photos was used to determine the studied terrains and scope.ً the regional Orthophotos (aerial photos which reduce the diversions related to camera and change in altitude after photography and then they are made after combination with digital elevation model by means of special software) with an accuracy of 1:2000 have been modelled incorporation with HEC-GeoRas extension and HEC-RAS software with return period of 25, 50 ,100 years. In order to achieve this we used Orthophotos in HEC-GeoRas extension to extract (access) the geometric features for 202 cross sections and calibration of hydraulic HEC-RAS model. We estimated water profile in return period of 25, 50, 100 years. Also we used HEC-GeoRas extension to show the flood zone of river. The results showed that there were no significant differences in flood zones In addition river condition in the mentioned return periods was subcritical.