The growth morphology and the electronic structures of thin metastable Ag films grown on Si(001)2×1 surfaces at low temperatures are investigated by scanning tunneling microscopy and angle-resolved photoemission spectroscopy using synchrotron radiation. The morphology of Ag films exhibits a strong thickness and substrate temperature dependence indicating an intriguing growth mechanism. At a nominal coverage larger than 5 ML, the as-deposited film is composed of homogeneous clusters having 3-dimensional character at the substrate temperatures of ∼100 K and of flat epitaxial Ag(111) films by a subsequent annealing at 300∼450 K. Discrete Ag 5 s states are observed at binding energies of 0.3∼3 eV together with the surface state. The discrete electronic states can be interpreted in terms of the quantum-well states (QWS) based on the phase-shift quantization. The phase shift, the energy dispersion and the thickness-versus-energy relation (Structure Plot) of the QWS are consistently derived. On the other hand, for the in-plane dispersion, in contrast to the free-electron-like behavior expected, these QWS show (i) a significant enhancement of the in-plane effective mass with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-Γ⁻ point. Such unexpected electronic properties of QWS are investigated in detail and found obviously related to the semiconductor substrate band structure. Furthermore, the QWS splitting is explained in terms of the energy dependence of phase shift at the film-substrate interface occurring at the substrate band edge.