Improvement of electrical properties by heat treatment with high-pressure H2O vapor was discussed for 7.4×1017cm−3 phosphorus-doped pulsed laser crystallized silicon films. The analysis of the electrical conductivity resulted in that 1.3×106-Pa-H2O vapor annealing at 270°C for 3 h reduced the density of defect states from 5.5 ×1018 cm−3(as crystallized) to 5.0×1017 cm−3 for tail states and from 1.0×1018 cm−3(as crystallized) to 3.0×1017 cm−3 for deep level defect states. The potential barrier height at grain boundaries decreased from 0.34 eV (as crystallized) to 0.05 eV by the heat treatment. The combination of oxygen plasma with high-pressure H2O vapor annealing effectively reduced the densities of defect tail sates as well as deep level defect states. It achieved a high performance of thin film transistors with a threshold voltage of 1.3 V and an effective mobility of 160 cm2/Vs.