Design flood hydrograph estimation is a key problem in hydrology and is necessary for a variety of applications from the design of hydraulic structures to flood risk mapping processes. Furthermore, in large ungauged basins (>1000 km²) design flood estimation methods are mainly rely on single-event theories using digital elevation models, land use / land cover and soil type data and relevant meteorological information (temperature and rainfall data). The single event-based deterministic approach is adopted, based on three modelling components: (i) synthetic storm generator; (ii) hydrological simulation model; and (iii) hydraulic routing model. In this study the 100-year design flood (which is assumed equal with the 100-year extreme rainfall) is estimated for Pinios river basin, Thessaly, Greece at Larissa outlet station (upstream area about 6500 km²). The hydrological approach is based on semi-distributed modelling of the rainfall-runoff process (at sub-basin scale) using the HEC-HMS software and the SCS-CN method for estimating rainfall excess, as well as the unit hydrograph theory for propagating the surface runoff to the subbasin outlets. Design Rainfall at subbasin scale is estimated from 13 IDF rainfall point curves using thiessen polygons and adjusted to the mean elevation of the subbasin with the developed precipitation gradients. The design flood hydrograph is estimated by combining the Intensity-Duration-Frequency (IDF) approach with standard time profiles, for constructing synthetic rainfall events of a certain probability, the SCS-CN method for extracting the excess from the gross rainfall, and the unit hydrograph theory, for propagating the surface runoff to the basin outlet.