By investigating ten typical magnetic clouds (MCs) associated with large geomagnetic storms (Dst $\leq-100$ nT) from 2000 to 2003, the geoeffectiveness of MCs with various orientations is addressed. It is found that the Dst peak values during the geomagnetic storms are well estimated by applying flux rope model to these magnetic clouds. A high correlation between estimated and observed Dst values is obtained. Moreover, the effect of orientations of MCs on intensities of geomagnetic storms is studied. It is found that the favorable orientations of MCs are approximately at $\theta\sim70^{\circ}$ and $\phi\sim40^{\circ}$ in GSE coordinates to cause large geomagnetic storms. Further, by analyzing solar observations of four associated erupted filaments, the question who determine the orientations of MCs is studied. The likelihood of predicting the intensities of a geomagnetic storms several tens hours before their occurrences is also discussed.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html