The effects of temperature and pressure upon structural, dynamical, and electronic properties of liquid H(2)O were systematically investigated via first principles molecular dynamics and in situ x-ray diffraction in the chosen isochore (1.00-1.61 g/cm(3)) and isotherm (300-900 K) conditions. Our study suggests that the crossover occurs between H-bonded and simple-liquidlike liquids by raising temperature in molecular liquid phase.