In this work, high quality yttrium-iron-garnet (YIG) thin films were fabricated on a Ga3Gd5O12 substrate by pulsed laser deposition (PLD). The material composition, iron ion valences and magnetic properties of the YIG films were systematically investigated as a function of annealing temperature. X-ray diffraction and Raman spectroscopy measurements demonstrate optimal YIG film crystallinity when annealed at 800 degrees C. X-ray photoelectron spectroscopy confirms the Fe2+ ions were strongly suppressed in YIG thin film after annealed at 800 degrees C. In this case, 60 nm-thick YIG films exhibit excellent magnetic properties having bulk-like saturated magnetization (1755 Gs), a minimized ferromagnetic resonance linewidth (8.3 Oe) and a low Gilbert damping parameter of 5.76 x 10(-4). Our study prove that suitable annealing conditions play a critical role on garnet composition, iron ion valences and magnetic properties for PLD grown high quality YIG thin films, which have great potential in the fabrication of low dissipative spintronic devices.