We have investigated the orientation and conformation of protein molecules at the polystyrene (PS)/protein solution interface using sum frequency generation (SFG) vibrational spectroscopy, supplemented by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). In this research, we studied fibrinogen as a model protein. SFG studies indicate that fibrinogen adopts a bent structure after adsorbing to the PS surface. A broad orientation distribution of fibrinogen coiled-coils at the interface has been quantified by combining SFG and ATR-FTIR measurements. Error analysis for such a deduced distribution was carried out. This research demonstrates that quantitative structural information such as orientational and conformational ordering of proteins at interfaces can be studied using SFG supplemented by other spectroscopic techniques.