Amorphous LaAlO3 (LAO) and LAOxNy (LAON) films have been prepared by pulsed laser deposition technique on Si (100) substrates and Pt coated silicon substrates. X-ray diffraction, transmission electron microscopy and differential thermal analysis investigations showed that both kinds of films remain amorphous up to a high temperature of 860℃. Atomic force microscopy study indicated that the surface of the deposited films is very smooth with a root mean square roughness of 0.14nm for 8nm LAO. LAON films have a smoother surface than that of LAO films. High-resolution transmission electron microscope studies showed there often exists interfacial reaction between LAO and Si. One LAON/Si structure nearly without interfacial layer has been obtained. For LAO films, high bandgap of 6.55eV and medium dielectric constant of 25-27 have been obtained. The LAON films showed small equivalent oxide thickness of 1.1nm with a low leakage of 0.074A/cm2@Vg511V. It is proposed that amorphous LAON films are very promising dielectric materials for high k gate dielectric applications.