An initial step in flood hazard mapping is hydrological modelling. We present a recent river flood modelling approach in Bosnia and Herzegovina (BiH) for small ungauged catchments of drainage area up to 32 km2. To estimate peak flow of required probability in small catchments, we use the rational method. The paper focus is GIS based procedure for producing the runoff coefficient map for BiH from the DTM and land cover map. For validation of the peak flow modelling results in small ungauged catchments we use diagrams of peak flow per catchment area (specific runoff) versus catchment area in medium and large gauged catchments. The results indicate agreement in specific runoff for 100 and 500 years return period compared to reference runoff in gauged catchments and a mild drop in specific runoff for 20 years.